Another similarity between pro- and anti-homosexual forces worldwide has been intense debate about scientific findings regarding sexual orientation. In the West, 2 there has been a strong correlation between political and moral positions about homosexuality and certain scientific positions about the causes of homosexuality. For example, in the United States, the National Association for Research & Therapy of Homosexuality, whose members believe that homosexuality is an undesirable and treatable condition, maintains a website that includes material attempting to debunk claims that sexual orientation is inborn. Scientific findings have often been used to either support or attack gay rights ( Horton, 1995 ; Pitman, 2011 ; Sprigg, 2012 ). In Uganda, anti-homosexual activists from the United States have been influential ( Gettleman, 2010 ; Throckmorton, 2010 ), and science has also figured prominently in their campaigns for anti-homosexual legislation. President Yoweri Museveni initially refused to sign the Anti-Homosexuality Act because he was unsure whether scientific evidence indicated that sexual orientation is either inborn or acquired. Only in the latter case did he believe that the anti-homosexual legislation could be justified ( Mugerwa, 2014 ). Accordingly, he asked for a summary of the relevant scientific evidence. The resulting statement provided by the Uganda Ministry of Health ( Aceng, 2014 ) removed Museveni’s doubts, and he signed the bill. The Constitutional Court in Uganda subsequently struck down the bill, and currently it is uncertain whether legislators will try again ( Feder, 2014 ; Williams, 2015 ).

It might be tempting to assume that much of the world is lagging behind but will ultimately follow the more accepting Western nations toward tolerance. That outcome is not assured, however. In some nations, tolerance of homosexuality appears to be decreasing. For example, Uganda has been struggling with the issue of whether to increase penalties for criminal offenses related to homosexuality. Same-sex relationships are already illegal in Uganda (as they are in most, but not all, African countries), with penalties as severe as 14 years in prison, but many Ugandan lawmakers have sought to increase penalties substantially. Early versions of proposed legislation included the death penalty for certain offenses (e.g., homosexual acts committed by a person with HIV).

The trajectory of LGB rights has been quite different in many other parts of the world, however. Currently, 75 countries legally proscribe homosexual behavior. Eleven countries—all in Africa, Asia and the Middle East—retain the death penalty as a possible sanction for homosexual acts 1 (International Lesbian and Gay Association, 2015; Stewart, 2015 ).

The political rights of lesbian, gay, and bisexual (LGB) men and women have dramatically improved in many Western countries during the past 50 years. In the United States, for example, the Supreme Court ruled in June 2015 that individual states must allow marriages between same-sex couples. This result would have been unthinkable in 1965, when homosexual behavior was illegal, homosexual inclinations were a source of shame, and most Americans believed homosexuality was a mental illness (e.g., “The Homosexual in America,” 1966 ). As of May 2015, 118 nations do not criminalize homosexual behavior ( Carroll & Itaborahy, 2015 ).

The most common meaningful controversy across time and place has concerned the extent to which homosexuality is socially influenced and, more specifically, whether or not it spreads as a result of contagion and social tolerance. There is no good evidence that either increases the rate of homosexual orientation, although tolerance may facilitate behavioral expression of homosexual desire. Suppressing homosexual behavior imposes an immense burden on homosexually oriented people and serves no apparent legitimate social goal that cannot be reached in other ways.

Scientists, activists, and policy makers should reason more carefully regarding potential ethical or policy implications of scientific findings. For example, the issue of whether sexual orientation is chosen represents intellectual confusion, and no scientific finding will illuminate this issue in any interesting way. Although clumsy reasoning may advantage a particular political position in the short term, in the long term, clear thinking is best for everyone.

Scientifically, sexual orientation is an important, fundamental trait that has been understudied because it is politically controversial. This is a mistake. In fact, the more politically controversial a topic, the more it is in the public interest to illuminate it in a revealing and unbiased manner. Our article is offered in the spirit of progress toward that end.

No causal theory of sexual orientation has yet gained widespread support. The most scientifically plausible causal hypotheses are difficult to test. However, there is considerably more evidence supporting nonsocial causes of sexual orientation than social causes. This evidence includes the cross-culturally robust finding that adult homosexuality is strongly related to childhood gender nonconformity; moderate genetic influences demonstrated in well-sampled twin studies; the cross-culturally robust fraternal-birth-order effect on male sexual orientation; and the finding that when infant boys are surgically and socially “changed” into girls, their eventual sexual orientation is unchanged (i.e., they remain sexually attracted to females). In contrast, evidence for the most commonly hypothesized social causes of homosexuality—sexual recruitment by homosexual adults, patterns of disordered parenting, or the influence of homosexual parents—is generally weak in magnitude and distorted by numerous confounding factors.

Individuals’ political attitudes about sexual orientation tend to correlate with their views of the causes of sexual orientation. Those who hold positive attitudes (i.e., that there is nothing inherently wrong with nonheterosexuality or its open expression) have tended to believe that sexual orientation is due to nonsocial causes such as genetics. Those who hold negative attitudes (i.e., that nonheterosexuality is undesirable or immoral and that society should restrict its free expression) have tended to believe that homosexuality has social causes, such as early sexual experiences and cultural acceptance of nonheterosexuality. We refer to these as the “nonsocial” and “social” hypotheses, respectively. Both hypotheses require direct scientific support; neither can claim confirmation solely because support for the other is weak.

Across many different cultures, male and female nonheterosexuality in adulthood tends to be preceded by childhood gender nonconformity: a pattern of behavior somewhat like that of the other sex. Childhood gender nonconformity is a matter of degree, and it can range from subtle to extreme. Often, it emerges at an early age, despite conventional gender socialization. Among some adults, childhood gender nonconformity appears to persist into adulthood.

Male and female sexual orientations differ in several respects. Women are more likely to report a bisexual than an exclusively same-sex orientation; men show the opposite pattern. Men’s sexual orientations are closely linked to their pattern of sexual arousal to male versus female erotic stimuli; women’s are not. Women appear more likely than men to experience same-sex attraction in the context of close affectionate relationships, and their patterns of sexual attraction appear more likely to exhibit change over time.

Sexual orientation refers to relative sexual attraction to men, to women, or to both. People who are sexually attracted to the same sex (whom we denote as “nonheterosexual”) represent a minority of adults. Those with predominantly same-sex attractions comprise fewer than 5% of respondents in most Western surveys. Data from non-Western cultures are consistent with this conclusion. There is no persuasive evidence that the rate of same-sex attraction has varied much across time or place.

This article has two related goals. The first is to provide a current summary of scientific findings regarding sexual orientation. Although we focus most on causation, we also address other scientific issues concerning sexual orientation, including its meaning and measurement, sex differences in its expression, its development, and its expression across time and place. Regarding causation, we provide a taxonomy of causal hypotheses and review evidence for them. These include hormonal, genetic, social environmental, and nonsocial environmental influences. Our second goal is less scientific and more analytical: to criticize and improve common but incorrect reasoning in this domain. For example, the commonly phrased question of whether sexual orientation is “a choice” is a poor one for advancing either scientific understanding or policy. A more meaningful formulation is whether sexual orientation is socially influenced.

The science of sexual orientation comprises a very large body of empirical findings, and so we must narrow it in two ways. First, we focus on research areas we believe are most relevant to the public interest, in both of the respects mentioned above. Second, acknowledging current valid concerns about the excess of statistically significant—but incorrect—scientific findings ( Simmons, Nelson, & Simonsohn, 2011 ), we have tried to focus on the most well-established findings. In addition, we focus on research that has garnered considerable public attention, such as Hamer, Hu, Magnuson, Hu, and Pattatucci’s (1993) genetic linkage study, LeVay’s (1991) brain study, and Regnerus’s (2012a) study of children with nonheterosexual parents. Although we are limiting our review in some ways, we are expanding it in others. Namely, we want to address not only the preoccupations of scientists but also those of non-academics interested in sexual-orientation science. With these goals in mind, we commence our scientific review.

The science of sexual orientation is in the public interest for at least two reasons. First, as we have noted, such science is frequently—if not always correctly—used to support political, social, and moral conclusions regarding homosexuality. Second, the science of sexual orientation—basic inquiry into why some people are sexually attracted to men and others to women—is intrinsically interesting. Because there is so much public interest in the science of sexual orientation, much relevant literature on this topic comes from sources outside of conventional scientific journals, such as news media, political advocacy writings, and blogs. We have freely cited these unconventional sources where relevant.

Regarding the second goal of this article, we believe that vast amounts of time have been wasted through the use of imprecise language and dubious arguments concerning the linkage of scientific findings on sexual orientation to political conclusions regarding LGB rights. We mean to correct the most common and serious linguistic and logical mistakes in this arena. More specifically, we argue below that the links between scientific findings and desirable social policies have often been overstated and misidentified. We expect our review will clarify which research questions are potentially politically important and which are “merely” scientifically important. We hope to eliminate, or at least to reduce, long-standing arguments that mix the wrong scientific and political questions. If we can do so, perhaps more progress can be made in resolving unavoidable rather than unnecessary conflict. Thus, we have ambitions to convey basic science accurately and to influence political discussions rationally.

Our article has two main goals: first, to review the current science of sexual orientation, and second, to consider the relevance of scientific findings to political debates about homosexuality. Regarding the first goal, we have focused most effort on the question of causation. The question of whether sexual orientation is influenced—and to what degree—by specific aspects of nature and nurture is the most important and contentious scientific question at issue. Yet we have also addressed several related scientific issues concerning sexual orientation, including its meaning, prevalence, sex differences, development, and universality.

After President Museveni’s call for scientific evidence about the causes of homosexuality, some of us were approached by persons hoping to affect the course of events in Uganda. They requested that we provide a scientific statement regarding the current status of the science of sexual orientation ( Throckmorton, 2014 ). The delay between Museveni’s reconsideration and his decision was brief, however, and not conducive to a thorough and considered scientific review. Subsequently, we decided to write this article, with the aim of providing one. Collectively, we are well positioned to write such an article, given that the five first authors conduct complementary programs of research that inform on the science of sexual orientation and the last author has published extensively on nonheterosexuality in Africa. We have not attempted to time the completion of the article to coincide with any particular decision by the Ugandan government, but we hope that policymakers in Uganda and elsewhere will find it useful. We expect some of the issues we write about to be discussed in political debates about homosexuality—both in Uganda and in the rest of the world—for the foreseeable future. 3

Finally, we note that there has been a tendency for LGB individuals and their advocates to favor higher prevalence figures, whereas those opposed to LGB rights have tended to favor lower figures (e.g., LaBarbera, 2014 ). To the extent that this dispute is political, it makes little sense. If homosexuality is wrong, then it is wrong even if it is common; if it is not wrong, then nonheterosexual people deserve their rights regardless of how rare they are. As we noted earlier, the conflation of political motives and scientific findings has been common in debates related to sexual orientation, and when this conflation is mistaken, it is to the detriment of both politics and science.

Although there may be scientific value in conducting future surveys of Western subjects to increase the precision of estimates related to the prevalence of nonheterosexual (and, necessarily, heterosexual) orientation, we do not see this as a high priority. There have already been a sufficient number of carefully sampled Western surveys related to sexual orientation, and hence future meta-analyses of these data may reveal interesting systematic patterns. We worry, however, that variation in prevalence estimates between studies may primarily reflect measurement error, both systematic and random. Asking increasingly detailed questions and perhaps even including non-self-report measures related to sexual orientation have the potential to reveal more than yet another carefully sampled self-report survey. Additionally, rather than continuing to survey the same, very similar, Western populations, it would be more scientifically useful to survey more non-Western populations.

Clearly, no one number can provide an estimate of the prevalence of nonheterosexual orientation. Even assessments of highly specific aspects of homosexuality have yielded a fairly wide range of estimates across recent studies conducted with similar populations. Several conclusions are possible, however, with a high degree of confidence. First, Kinsey’s famous survey likely overestimated the frequencies of nonheterosexual attractions and experiences. Second, individuals with incidental homosexual feelings and contacts are much more common than those with substantial (i.e., persistent and strong) feelings and frequent same-sex experiences. Third, individuals with substantial homosexual feelings comprise a small, albeit nontrivial, minority of adults in Western developed nations—a smaller percentage than suggested by Kinsey’s data from more than half a century ago—despite the remarkable increase in tolerance of homosexuality ( Voeten, 2012 ).

Depending on what one means by “nonheterosexual,” nonheterosexual identity may be a conservative measure of nonheterosexual orientation. In the recent review, three studies assessed same-sex attraction as well as identity. The percentage of adults who admitted to “any homosexual feelings” ranged from 1.8% to 11%, exceeding the percentage identifying as “homosexual” or “bisexual” by factors ranging from 1.5 to 3.1 ( Gates, 2011 ); note the wide range of the estimates. But what does it mean to say that one has experienced same-sex attraction “at least once” ( Smith, Rissel, Richters, Grulich, & Visser, 2003 ) or that one’s sexual attractions are “mostly” directed toward one sex ( Chandra et al., 2011 )? In the study that yielded a figure of 11% for a history of any homosexual attractions, only 3.3% of respondents said they were as attracted to the same sex as to the other sex. Are people who say that they have had at least one but possibly very few same-sex attractions intermediate between exclusively heterosexual and homosexual people on a continuum of sexual orientation? Or, alternatively, are they understanding the term “same-sex attractions” differently than most people? We defer consideration of these questions to our section on bisexuality. The percentage of adults who have ever had a homosexual experience is also larger than the percentage of adults who identify as homosexual or bisexual. In the recent review by Gates, three studies assessed both nonheterosexual identity and behavior. The percentage of adults reporting a history of any same-sex sexual interaction ranged from 6.9% to 8.8%, exceeding those reporting a nonheterosexual identity by ratios ranging from 2:3 to 3:3 ( Gates, 2011 ).

Starting in the 1980s and motivated by epidemiologists’ need for better numbers to monitor the AIDS epidemic, several large and careful surveys of sexual behavior have been conducted, primarily in North America, Europe, and Australia. Most of these assessed aspects of homosexuality as well as heterosexuality, and most focused on sexual behavior rather than sexual attraction. Results of these studies are generally consistent in providing far lower numbers than Kinsey’s 10%. Asking about sexual identity—whether respondents consider themselves homosexual/gay/lesbian, bisexual, or heterosexual/straight—is perhaps the simplest way to survey people about sexual orientation. A recent survey of 34,557 U.S. adults yielded rates of 96.6% heterosexual, 1.6% gay or lesbian, and 0.7% bisexual ( Ward, Dahlhamer, Galinsky, & Joestl, 2014 ). Additionally, 1.1% of respondents identified as “something else” or said they “don’t know the answer.” These numbers are in reasonably close agreement with a recent review of nine large, careful studies conducted in Western populations ( Gates, 2011 ), which concluded that approximately 3.5% of U.S. adults identified as gay, lesbian, or bisexual. The only careful estimation of nonheterosexual orientation for a non-Western culture focused on Samoan males, and the resulting estimate of 1.4% to 4.7% for androphilia is similar to Western estimates ( VanderLaan, Forrester, Petterson, & Vasey, 2013 ).

Kinsey conducted the first large surveys of homosexuality in the United States during the 1940s ( Kinsey et al., 1948 ). His results shocked readers because they made homosexual behavior and attractions appear so common. For example, 37% of men surveyed admitted having had a homosexual experience. Most of these occurred during adolescence, perhaps indicating brief experimentation. Approximately 10% of the men had been more or less exclusively homosexual for at least 3 years during adulthood—this is the origin of the “10% of people are homosexual” assertion that was commonly made until recent and more representative surveys supported lower rates. About 4% of his male respondents had been homosexual for their entire lives.

One of the most common questions scientists are asked concerning sexual orientation concerns the population prevalence of homosexuality and bisexuality. It is impossible to provide precise estimates, for several reasons. First, the different phenomena associated with homosexuality and bisexuality—behavior, identity, and sexual orientation—vary in frequency. For example, people who identify as heterosexual may still engage in homosexual sex and admit homosexual attraction, and hence one would find different population estimates depending on which phenomenon one assessed. Second, the different phenomena associated with homosexuality and bisexuality may vary over the life course, and hence one would find different population estimates depending on whether one assessed individuals’ current patterns of behavior and attractions versus their total lifetime history of behavior or attractions. For example, the percentage of people who have ever had a homosexual experience is larger than the percentage of people who have had one during the past year, which, in turn, is larger than the percentage of people who have had only homosexual experiences for their entire lives. Third, homosexuality remains stigmatized to some degree even in the most liberal nations ( Kohut, 2014 ), and thus some individuals may be motivated to underreport homosexual attractions, identity, and behavior. Fourth, because some aspects of homosexuality—especially homosexual identity and exclusively homosexual attractions—are uncommon, precision requires large and representative samples, which are expensive and difficult to survey. Finally, there is no good reason to expect that a single set of estimated frequencies applies to all places and times. This is especially likely to be true for sexual identity and behavior, which seem to us far more culturally malleable than sexual attraction.

During the past decade, several other measures of sexual orientation have been studied that do not rely on self-report. These include viewing time (time spent viewing pictures of males vs. females or rating them for attractiveness; Israel & Strassberg, 2009 ; Rullo, Strassberg, & Israel, 2010 ), fMRI activation (activation of relevant brain areas in response to viewing male versus female erotic images; Safron et al., 2007 ), implicit attitudes ( Snowden, Wichter, & Gray, 2008 ), and pupil dilation (pupil dilation while viewing pictures or videos of males vs. females; Rieger & Savin-Williams, 2012 ). Studies using these measures have yielded strong correlations with self-reported sexual orientation in samples of individuals with no apparent reason to give inaccurate self-reports. These measures are more onerous to administer than self-report measures, but some have the advantage of being less onerous than genital arousal measures. Many of these newer measures are probably less objectionable than invasive genital measures to many potential research subjects, and in that sense they are easier to use. All require considerably more research to understand their strengths and weaknesses compared with genital and self-reported arousal and with each other.

Measures of female genital arousal also exist. The most common uses vaginal photoplethysmography (VPP), a technique that is sensitive to changes in blood flow in the vagina. Women’s genitals, like men’s, have increased blood flow during sexual arousal. Women’s patterns of genital arousal to male versus female sexual stimuli do not mirror those of men, and they are different in a way that prevents VPP from being useful in measuring female sexual orientation. We discuss this evidence below in the “Sex Differences in Expression of Sexual Orientation” section.

When a man’s PPG-assessed arousal pattern differs from his self-reported sexual orientation, what should we conclude? In some cases, PPG measurement is poor, generally as a result of low levels of induced erection, and in such cases it would be rash to dismiss a man’s self-report and better to ignore the PPG data. However, in cases in which PPG arousal is very well measured—meaning at least that a man produces a clear pattern of arousal that is potentially repeatable—PPG arousal is the better measure. This is especially true when there is reason to doubt a man’s self-report. For this reason, PPG arousal measures are important, both conceptually and scientifically, in examining certain issues related to sexual orientation.

Although Freund invented PPG to assess male sexual orientation as we mean it here (i.e., homosexual vs. heterosexual orientation), that application was never common and has become less so. Use of PPG to assess typical male sexual orientation is almost exclusively done in the context of basic scientific research (i.e., research aiming to answer scientific questions as opposed to applied research on clinical populations, which has the goal of developing useful treatments or assessments), and below we review some important findings from research using that technique. The most common practical applications of PPG assessment have been in the area of diagnosis and treatment of erectile dysfunction ( Broderick, 1998 ) and detection of pedophilia, typically among men accused or convicted of sex offenses (e.g., Blanchard, Klassen, Dickey, Kuban, & Blak, 2001 ). One general implication of this work is that PPG can be useful in assessing the sexual interests of men who wish to hide them. In studies employing PPG of normal men who vary in sexual orientation—men who presumably have nothing to hide—very high correlations are generally obtained between genital and self-report measures (e.g., Chivers, Rieger, Latty, & Bailey, 2004 ).

PPG-measured arousal patterns are considered homosexual (or androphilic) when a man’s arousal to adult male stimuli substantially exceeds his arousal to adult female stimuli and heterosexual (or gynephilic) when the opposite pattern occurs. Bisexual arousal patterns do not necessarily imply equal levels of arousal to male and female stimuli. Rather, a man is considered to have a bisexual arousal pattern when the absolute difference between his arousal to male versus female stimuli is smaller than the absolute difference observed among heterosexual and homosexual men. To clarify, a homosexual man is typically much more aroused by male than female stimuli, and a heterosexual man is typically much more aroused by female than male stimuli. Both of these patterns yield large absolute differences between arousal to female versus male stimuli. A bisexual man, in contrast, should have levels of arousal to female and male stimuli that are not as discrepant, yielding a smaller absolute difference.

Pioneering sex researcher Kurt Freund invented a technique for assessing penile erection in response to different kinds of sexual stimuli as a window on men’s sexual orientation ( Freund, 1963 ). General terms for the measurement of penile erection include penile plethysmography (PPG). Freund’s particular method used pictures of nude men and women as stimuli and a barometer-like contraption placed over men’s genitals to measure their erections via changes in air pressure. Freund’s technology is sensitive to small increases in penile erection, but it is onerous—for example, it requires a technician to place the instrument. Thus, most research on male sexual orientation with genital measurement has employed circumferential PPG measures, such as the penile strain gauge, which are somewhat less sensitive to small changes in penile erection but less difficult to use ( Janssen, 2002 ; Kuban, Barbaree, & Blanchard, 1999 ). Increasingly, researchers have begun to use videos rather than still images as stimuli, because the former evoke more arousal and thus allow better measurement.

Although Kinsey was ultimately making an empirical claim requiring scientific evidence, his scale provided a simple way of collecting useful data, and it has become widely used. It is important to note that the Kinsey Scale is a self-report instrument. This makes sense and is often optimal—who better to ask about people’s sexual orientations than the people we want to know about?—but self-reported Kinsey Scale scores can sometimes be inaccurate or incomplete, especially when people want to conceal their sexual orientations or when they are confused or conflicted about their sexual feelings.

Males do not represent two discrete populations, heterosexual and homosexual. The world is not to be divided into sheep and goats. Not all things are black nor all things white. It is a fundamental of taxonomy that nature rarely deals with discrete categories. Only the human mind invents categories and tries to force facts into separated pigeon-holes. The living world is a continuum in each and every one of its aspects. The sooner we learn this concerning human sexual behavior, the sooner we shall reach a sound understanding of the realities of sex. ( Kinsey, Pomeroy, & Martin, 1948 , pp. 638–639).

The Heterosexual-Homosexual Rating Scale, commonly referred to as the Kinsey Scale, is the best-known self-report measure of sexual orientation. The scale ranges from 0 (representing entirely heterosexual orientation) to 6 (representing entirely homosexual orientation); the middle score of 3 represents a bisexual orientation with equal attraction to men and women. 6 The other scores represent gradations between those anchors. Kinsey famously justified his measure of sexual orientation by asserting,

Many scientific studies related to sexual orientation have compared subjects who have been recruited on the basis of identifying as either exclusively homosexual/gay or exclusively heterosexual/straight. That is, such studies have ignored or even excluded bisexually attracted individuals. On the one hand, this approach to studying sexual orientation is incomplete. On the other hand, findings from studies using this approach need not be misleading, provided we acknowledge their limitations.

Sexual orientation is defined here as attraction to members of the same sex, both sexes, or the other sex. Most researchers studying sexual orientation focus on self-reported patterns of sexual attraction rather than sexual behavior or identity, because sexual behavior and identity can be extremely constrained by local culture and because sexual attraction motivates behavior and identity, rather than vice versa.

Although the four aforementioned phenomena of sexual orientation (behavior, attraction, identity, and arousal) tend to go together—homosexually oriented persons tend to identify as gay or lesbian and to have sex with same-sex partners—they do not always. For example, some men who identify as straight/heterosexual have sex with other men and appear to be most strongly attracted to men. Some adolescents engage in homosexual activity yet grow up to identify and behave as heterosexuals. Similarly, some individuals pursue same-sex relationships in sex-segregated environments, such as boarding schools, prisons, or the military, but resume heterosexual relationships once other-sex partners are available. Moreover, the degree of association among homosexual attraction, behavior, and identity varies across individuals in different cultural contexts. For example, in some cultures and communities, homosexually attracted men regularly engage in same-sex behavior while still maintaining a heterosexual identity. In other cultures and communities, such a pattern may be less common, and homosexually attracted men may find it difficult to find male partners without identifying themselves as homosexual or bisexual.

Terminology also differs among the different phenomena of sexual orientation. People identify as “gay,” “lesbian,” “bisexual,” or “straight.” Scientists primarily concerned with the consequences of same-sex behavior may refer to “men who have sex with men.” In this report, we refer to sexual attraction patterns as “homosexual,” “bisexual,” or “heterosexual.” 5 Alternatively, it is sometimes more relevant to distinguish individuals not according to whether they are attracted to same-sex or other-sex partners but according to whether they are attracted to men ( androphilic ) or women ( gynephilic ). In this usage, both heterosexual women and homosexual men would be considered androphilic because both groups are attracted to men; both heterosexual men and homosexual women would be considered gynephilic because both groups are attracted to women.

Four related phenomena fall under the general rubric of sexual orientation, but they are conceptually and empirically distinguishable. They are listed here not in order of importance but in an order that reflects their degree of historical attention. The first phenomenon, sexual behavior , consists of sexual interactions between persons of the same sex (homosexual), the other sex (heterosexual), or both sexes (bisexual). The second phenomenon, sexual identity , is one’s self-conception (sometimes disclosed to others and sometimes not) as a homosexual, bisexual, or heterosexual person. The third phenomenon of sexual orientation is one’s degree of sexual attraction to the same sex, both sexes, or the other sex. The fourth phenomenon is one’s relative physiological sexual arousal to men versus women (or to male vs. female erotic stimuli), which is more closely related to other aspects of sexual orientation in men than in women. 4

What accounts for the ability of some women to be sexually fluid? L. M. Diamond (2003b) has theorized that to an extent, romantic love and sexual desire rely on different motivational systems: the former more on the attachment or pair-bonding system, the latter more on the sexual mating system. Although separate, these two systems may affect each other, and the bidirectional links (permitting feelings of strong emotional attachment to give rise to sexual desire) may be especially strong in women. Another possibility concerns sex differences in the reproductive context of female versus male sexual behavior. Whereas all of men’s sexual interactions can result in reproduction, women’s sexual behavior can result in reproduction only if it coincides with ovulation. However, women are capable of becoming sexually aroused at any point in the menstrual cycle, and although their sexual motivation appears to increase as a function of the rising estrogen levels that accompany ovulation, their sexual arousability during the rest of the month appears unrelated to estrogen levels (reviewed in Wallen, 1995 ). Women’s capacity for arousability independent of ovulation permits them to experience sexual desires and behaviors with same-sex individuals at no evolutionary cost. In fact, researchers have argued that sexual behavior with nonreproductive (and even same-sex) partners can confer multiple evolutionary advantages on females, such as alliance formation ( de Waal, 1987 ; Hohmann & Fruth, 2000 ). Hence, one possibility is that women’s capacity for sexual fluidity evolved in concert with (or is an artifact of) their capacity for nonreproductive arousability during nonfertile parts of the menstrual cycle.

Regardless of the reason for sex differences in category-specificity, this sex difference is likely to produce additional sex differences in sexuality. Sexual arousal is a powerful motivating force, and so men’s greater category-specificity might be expected to produce more categorical patterns of sexual behavior (i.e., exclusively heterosexual behavior among heterosexual men and exclusively homosexual behavior among homosexual men). In contrast, the fact that women’s genital arousal is less category-specific might be expected to produce less categorical (and hence more “bisexual”) patterns of sexual behavior, which may help to explain the aforementioned sex difference in bisexual identification.

Another possibility is that the difference is based in fundamental evolved differences between female and male sexuality ( Baumeister, 2000 ; Chivers & Bailey, 2005 ), although currently there is no direct evidence speaking to this question. Intriguingly, Goy and Goldfoot (1975) showed over 30 years ago that in many different mammalian species, bisexuality is an intrinsically dimorphic trait that develops (through prenatal hormonal pathways) in either the male or the female of a species, but never both. This suggests the provocative possibility that in humans, women are “the more bisexual sex,” whereas males are more likely to be exclusively heterosexual or homosexual.

Accepting for now the validity of the hypothesis that women’s sexual arousal patterns are less category-specific than men’s, the question arises as to why this is the case. One possibility is cultural—namely, Western cultures produce omnipresent depictions of female beauty, which are often sexualized, and exposure to these images from an early age may sensitize both men and women to experience sexual arousal to the female body. Although this explanation might account for heterosexual women’s genital arousal to female stimuli, it cannot account for the fact that homosexual men, who have experienced just as much exposure to sexualized images of women, do not experience sexual arousal to female stimuli. Similarly, exposure to sexualized images of women fails to account for the fact that homosexual women show more genital arousal to male sexual stimuli than homosexual men show to female sexual stimuli.

There are important differences between measures of male and female genital arousal that need to be considered before accepting the idea that women are less category-specific than men in their sexual arousal patterns. Female genital arousal is measured much differently than male genital arousal (specifically, a device is inserted into the vagina to measure changes in blood flow, in contrast to the erectile measure used for men). Nonetheless, several findings argue against the possibility that the sex difference in category-specific genital arousal is a measurement artifact. First, a study of postoperative male-to-female transsexuals (assessed with VPP) produced a category-specific pattern of genital arousal similar to that of men ( Chivers et al., 2004 ). This would not have been expected to occur if the vaginal measure of arousal was not effective in detecting category-specific arousal responses. Second, women also show substantially less category-specificity in their self-reported patterns of sexual arousal to male and female stimuli. Third, other correlates of sexual interest have been shown to be less category-specific in women than in men, such as viewing time for female and male sexual stimuli ( Lippa, Patterson, & Marelich, 2010 ), pupil dilation in response to such stimuli ( Rieger & Savin-Williams, 2012 ), and fMRI responses to such stimuli ( Sylva et al., 2013 ). 7

Women show a much different pattern of linkage between sexual orientation and genital arousal (i.e., genital blood flow as measured by VPP). In the laboratory, heterosexual women show approximately equal levels of genital arousal to male and female stimuli (despite sometimes reporting that they find male sexual stimuli more arousing). Homosexual women show a pattern of genital sexual arousal that is slightly category-specific (i.e., they show stronger genital arousal for female than male stimuli), but their genital responses are not as starkly category-specific as men’s ( Chivers et al., 2004 ; Chivers, Seto, & Blanchard, 2007 ). One potential interpretation of the sex difference in genital arousal patterns is that female sexual orientation is fundamentally less category-specific than male sexual orientation (i.e., that homosexual women retain some potential for attraction/arousal to men and heterosexual women retain some potential for attraction/arousal to women, even if they are unaware of this potential). This explanation presumes that a woman’s pattern of genital arousal provides a reliable measure of her sexual orientation (as is the case for men). An alternative explanation is that genital arousal is less reliably linked to sexual orientation in women than in men. This explanation remains agnostic as to whether female sexual orientation is less categorical in women than in men; it simply suggests that the measure of genital arousal commonly employed—namely, genital blood flow as assessed via VPP—cannot definitively answer this question, because this measure has a weaker pattern of association with female than male sexual orientation.

Sexual arousal comprises both subjective and genital response to a sexual stimulus. In men, sexual orientation is closely related to a pattern of genital sexual arousal. Specifically, homosexual men show genital arousal to men (or, in the lab, sexual stimuli depicting men), whereas heterosexual men show genital arousal to women (or, in the lab, sexual stimuli depicting women). This pattern has been called “category-specific” because men’s genital sexual arousal is specific to the category of person to whom they are most attracted ( Chivers et al., 2004 ). Indeed, one of us has argued that male sexual orientation can be precisely defined as a pattern of sexual arousal, for two reasons ( Bailey, 2009 ). First, as we have mentioned, sexual arousal patterns reliably reflect male sexual orientation. In cases where a man’s self-reported sexual orientation diverges from his sexual arousal pattern (and in which his sexual arousal pattern is robust and dichotomous, consisting of strong erections to one sex but not the other), we believe his sexual arousal pattern best represents his sexual orientation—if not necessarily his self-chosen sexual identity. Second, and more important, is the direction of causation. Sexual arousal pattern motivates sexual behavior, which provides most individuals with the fundamental basis for their sexual self-identification. In contrast, there is no evidence to suggest that individuals can consciously alter their genital arousal to fit a certain sexual identity label.

In summarizing recent prevalence figures for nonheterosexual orientation, we deferred discussing a consistent, large, and interesting sex difference. Among those who identify as nonheterosexual, women tend to identify as bisexual whereas men tend to identify as homosexual ( Gates, 2011 ). Studies that have assessed sexual attractions have found similar results. Figure 1 summarizes the distributions from a recent review of sexual orientation among 71,190 adult men and 117,717 adult women who reported their sexual orientations using a 5-point scale: 1 = entirely heterosexual/straight , 2 = mostly heterosexual/straight , 3 = bisexual , 4 = mostly homosexual/gay/lesbian , 5 = entirely homosexual/gay/lesbian ( Savin-Williams & Vrangalova, 2013 ). The figure reveals markedly different shapes for the two sexes. The large majority of both sexes rated themselves as “entirely heterosexual,” although men (93.2%) were considerably more likely to do so than women (86.8%). Many more women than men rated themselves as “mostly heterosexual.” For women, there is a steep decline in the frequency of “mostly heterosexual” to “bisexual” ratings, and then a more gradual decline to “mostly homosexual” and “entirely homosexual” ratings, which have similar frequencies. For men, the least frequently self-rated category is “bisexual,” with a slight rise in the frequency of “mostly homosexual” and “entirely homosexual” ratings. The male distribution is bimodal. Bimodal distributions are rare and potentially interesting, suggesting that two groups are different in kind and not merely degree. For example, a bimodal distribution would be expected if there were a single cause underlying the distinction between two groups, whereas a continuous distribution would be expected if there were multiple causes. The classic human example is biological sex, which is caused by a dichotomous genotype (XX vs. XY) that leads to a bimodal distribution of prenatal testosterone, generating categorical sex differences in some anatomical features, such as genitalia. One conclusion from the sex difference in distributions of sexual orientation is clear, however: Among the Western populations that have been studied, bisexual patterns of attraction are rarer in men than in women, and exclusively homosexual attractions are rarer in women than in men. The reasons for this sex difference are not known. It might reflect prenatal biological sex differences; it might reflect the influence of cultural factors that create different social contexts for the development of female versus male same-sex sexuality; it might reflect differences in women’s and men’s susceptibility to such social influences ( Baumeister, 2000 ); or it might simply result from measurement error. If there are innate sex differences in the causation of sexual orientation, then we should expect to find that the correlates of sexual orientation differ considerably between men and women. This is the topic we address next.

Before leaving the topic of gender nonconformity, we address a commonly raised question: Might the gender-atypicality of adult homosexual men and women simply reflect a culturally influenced self-fulfilling prophecy? In other words, given that society expects homosexual individuals to be gender atypical, and given that LGB communities often support and facetiously celebrate such gender atypicality, perhaps some homosexual people adopt gender-atypical characteristics to conform to their own stereotypes. Because of the evidence we have reviewed—indicating that gender nonconformity often begins before a prehomosexual child even has a sexual orientation or is aware of cultural stereotypes, and that the link between gender nonconformity and nonheterosexual orientation has been found in a wide variety of cultures—we think it is highly unlikely that gender nonconformity in LGB populations represents a self-fulfilling prophecy due to cultural beliefs. It is possible, however, that cultural stereotypes sometimes amplify gender nonconformity among LGB people. Many LGB individuals report that they have always been fairly gender-typical in dress, appearance, and interests. It is possible that as these individuals come to identify as LGB and participate in the LGB community, they adopt aspects of gender-atypicality.

It would be surprising if the large orientation differences in childhood gender nonconformity disappeared without a trace by adulthood. Several differences between heterosexual and nonheterosexual adults persist. They include patterns of occupational and recreational interests ( Lippa, 2005a , 2005b ). Research indicates that heterosexual men have greater interest in occupations and hobbies focusing on things and less interest in those focusing on people, compared with heterosexual women ( Lippa, 2005a ; Su, Rounds, & Armstrong, 2009 ). In contrast, homosexual men show a somewhat feminine pattern of interests, and homosexual women a somewhat masculine one. The orientation differences are large, although smaller than the sex differences. They are consistent with stereotypes about occupational differences between homosexual and heterosexual people. This makes them more difficult to interpret than childhood differences because both homosexual and heterosexual adults may—either consciously or unconsciously—mold their behavior in accordance with societal expectations.

Bisexual-identified individuals typically experience later recognition of same-sex attractions compared with homosexual men and women. This may reflect the fact that bisexual identity usually follows the establishment of a heterosexual identity ( Fox, 1995 ; Weinberg, Williams, & Pryor, 1994 ). Homosexual people are atypical in two ways: their lack of attraction to the other sex and their increased attraction to their own. In contrast, bisexual people are atypical in only the latter sense, and this may delay their recognition of their own difference.

How do nonheterosexual people experience the recognition of homosexual feelings? In a retrospective study, homosexual men reported that their early sexual attractions were not necessarily a source of any distress ( Savin-Williams, 1996 ). Same-sex attractions were often experienced as an obsession with being near masculine, often older, same-sex teenagers and adults, such as male teachers and coaches. Eventually, these men recognized that their same-sex desires were rarely shared by others, were not condoned, and should be hidden. Although first homosexual male experiences can occur in the context of romantic relationships, it is not uncommon for these experiences to occur with strangers ( Savin-Williams & Diamond, 2000 ).

Studies have shown that subjects recall first having feelings of sexual attraction at age 10, on average ( McClintock & Herdt, 1996 ). Male and female subjects report similar ages. Importantly, so do homosexual and heterosexual subjects. Age 10 is several years before the typical age of onset of sexual activity ( Cavazos-Rehg et al., 2009 ). This is consistent with an earlier retrospective study showing that homosexual men and women recalled their first homosexual feelings as preceding their first homosexual experiences by 3 years ( Bell, Weinberg, & Hammersmith, 1981 ). Although this gap may have changed since that publication, the important point—to which we return—is that the large majority of nonheterosexual people recall that homosexual desires preceded homosexual experiences.

Children cannot know their sexual orientations until they experience sexual attraction toward the same or the other sex. When does this happen? At present in North America, it is impossible to study actual children as they become sexually aware. Such a study would be a political minefield if not an ethical one. Therefore, the onset of sexual attraction has been studied using retrospective reports of adolescents and adults.

Both retrospective and prospective studies support a strong association between childhood gender nonconformity and adult nonheterosexuality. This does not mean, of course, that all nonheterosexual individuals were more gender nonconforming than average, much less that they were all extremely gender nonconforming. There are at least three potentially important implications, however. First, long before sexual attraction emerges, some children who will become nonheterosexual are markedly different than other children. Indeed, childhood gender nonconformity is often evident by age 2 ( Cohen-Kettenis & Pfäfflin, 2003 ). Second, there is little evidence that gender nonconforming children have been encouraged or taught to behave that way; rather, childhood gender nonconformity typically emerges despite conventional socialization. Third, the differences between children who will become nonheterosexual and those who will become heterosexual are related to sex-typed behavior and gender identity ( Bailey & Zucker, 1995 ; Zucker & Bradley, 1995 ). The content of these differences provides some potential clues about the causes of sexual orientation, which we discuss further below.

Several prospective studies of highly gender nonconforming children have been conducted, most focusing on boys (for reviews, see Bailey & Zucker, 1995 ; Zucker, 2014 ). These studies have identified children through clinical interventions to help the children accept their birth sex identity, as opposed to obtaining sex reassignment surgery in adulthood. Thus, in general, these children exhibited extreme gender nonconformity as well as discomfort with or confusion about their gender identity. For males, all studies have found most men to be homosexually or bisexually oriented at follow-up, 8 with rates ranging from 64% ( N = 129; Singh, 2012 ) to 80% ( N = 66; Green, 1987 ). The two studies including females were smaller and had more variable outcomes: 32% of 25 subjects ( Drummond, Bradley, Peterson-Badali, & Zucker, 2008 ) and 100% of 10 subjects ( Wallien & Cohen-Kettenis, 2008 ) endorsed bisexual or homosexual feelings. Not all of the studies had control groups, but all the rates of nonheterosexual outcomes far exceed plausible epidemiological estimates for the general population, which might be generously estimated at 5% (see Fig. 1 ). Thus, the prospective studies also suggest a large association between (extreme) childhood gender nonconformity and adult nonheterosexuality. In nearly all the studies, there was a significant tendency for subjects self-reporting as heterosexual to be younger than those self-reporting as nonheterosexual. This suggests that the prospective studies may underestimate the likelihood of a nonheterosexual outcome because younger nonheterosexual individuals are more likely to provide false claims that they are heterosexual ( Bailey & Zucker, 1995 ).

A 1995 review of retrospective studies found large differences in retrospectively reported childhood gender nonconformity between homosexual and heterosexual men (32 studies; d = 1.3) and women (16 studies; d = 1.0; Bailey & Zucker, 1995 ). The review estimated that with respect to male sexual orientation, 89% of homosexual men exceeded the heterosexual median score, whereas 2% of heterosexual men exceeded the homosexual median. For female sexual orientation, the respective figures were 81% and 12%. Although this review was published 20 years ago, there is little reason to suspect that its results would change much if updated. Furthermore, despite the skepticism of some writers (e.g., Fausto-Sterling, 2014 ), no study has persuasively demonstrated that retrospective studies exaggerate orientation differences associated with childhood gender nonconformity. Retrospective self-report measures have been supplemented by childhood home videos provided by heterosexual and homosexual adults, with similar results: Viewers of the videos could tell, at far better than chance levels, which children would grow up to be homosexual and which would grow up to be heterosexual ( Rieger, Linsenmeier, Gygax, & Bailey, 2008 ). The retrospective studies suggest that some degree of childhood gender nonconformity is a common precursor of adult homosexuality in both sexes. This conclusion holds for both Western and a wide array of non-Western cultures, including those in Asia, Latin America, Polynesia, and the Middle East ( Bartlett & Vasey, 2006 ; Cardoso, 2005 , 2009 ; Vasey, VanderLaan, Gothreau, & Bartlett, 2011 ; Whitam & Mathy, 1986 ).

Children who grow up to be nonheterosexual are substantially more gender nonconforming, on average, than children who grow up to be heterosexual. There are two types of studies supporting this. In retrospective studies, homosexual and heterosexual (and sometimes bisexual) adults are asked about their childhood behavior. In prospective studies, extremely gender nonconforming children are followed into adulthood to assess their sexual orientations. Both kinds of studies have limitations that in isolation might lead to skepticism about their validity. Retrospective studies rely on childhood memories, which might be distorted in a way that supports stereotypes about homosexuality and heterosexuality. Prospective studies often focus on highly unusual children, and one might worry about generalizing findings from these studies to less atypical children. Nonetheless, the two kinds of studies have produced highly convergent findings that support a strong association between childhood gender nonconformity and adult nonheterosexuality.

Childhood gender nonconformity—behaving like the other sex—is a strong correlate of adult sexual orientation that has been consistently and repeatedly replicated ( Bailey & Zucker, 1995 ). More specifically, childhood gender nonconformity comprises the following phenomena among boys: cross-dressing, desiring to have long hair, playing with dolls, disliking competitive sports and rough play, preferring girls as playmates, exhibiting elevated separation anxiety, and desiring to be—or believing that one is—a girl. In girls, gender nonconformity comprises dressing like and playing with boys, showing interest in competitive sports and rough play, lacking interest in conventionally female toys such as dolls and makeup, and desiring to be a boy. Childhood gender nonconformity usually emerges by preschool age. It is important to add that children are usually considered to be gender-nonconforming only if they persistently engage in a variety of these behaviors, as opposed to engaging in a single behavior once or twice. Furthermore, childhood gender nonconformity is not an either/or trait but, rather, a dimensional one, so the differences we discuss are a matter of degree and not of kind.

The most defensible conclusions from this work are that some bisexual-identified men have bisexual genital arousal patterns and some do not. This provides strong evidence against the (often stereotyped) notion that male bisexuality does not exist and that all bisexual-identified males are misrepresenting their sexual orientation. Yet basic questions regarding the prevalence of female and male bisexual phenomena and the specific differences between bisexual and homosexual phenomena remain unanswered; this remains an active topic of research.

Three studies have examined the genital arousal patterns of bisexual, heterosexual, and homosexual men. Two found that bisexual men’s self-reported subjective sexual arousal patterns were relatively bisexual, compared with those of monosexuals, but that their genital arousal patterns were not ( Cerny & Janssen, 2011 ; Rieger et al., 2005 ). The other study found statistically robust and strong effects for both subjective and genital measures, supporting the hypothesis that bisexual men have relatively bisexual arousal patterns ( Rosenthal, Sylva, Safron, & Bailey, 2011 , 2012 ). Which finding is correct? Likely, both are true, each for a different subset of bisexual-identified men. The studies that failed to find correspondence between bisexual identity and arousal patterns used relatively liberal inclusion criteria for bisexuality—namely, that men considered themselves bisexual and had self-reported Kinsey Scale scores in the bisexual range. In contrast, the study that found correspondence required bisexual men to have had both sexual and romantic experiences with members of both sexes (which may have more effectively excluded men whose orientations were more monosexual in nature despite their identification as bisexual).

To assess bisexual patterns of genital arousal in men, it is first necessary to decide what such an arousal pattern should look like, relative to a monosexual pattern of arousal. One possibility is that individuals with bisexual arousal patterns should show less of a difference (compared to monosexuals) between their arousal responses to men versus women ( Rosenthal, Sylva, Safron, & Bailey, 2012 ). Another approach emphasizes the overall degree of arousal to one’s “less preferred” sex (Rieger, Chivers, & Bailey, 2005 ). Consider the first approach. To the extent that bisexual-identified men have bisexual arousal patterns, they should have smaller absolute differences between their arousal to male and female stimuli compared with monosexual men. That is, a bisexual arousal pattern implies a relatively similar degree of arousal to male and female erotic stimuli. The rationale for this approach, which focuses on arousal to the less-preferred sex, is that a bisexual man should show greater arousal to female stimuli than would be shown by homosexual men as well as greater arousal to male stimuli than generally shown by heterosexual men. Thus, a bisexual man’s arousal to stimuli depicting his less-arousing sex—whichever that is—should be greater than that for monosexual men. The dependent variables using the two approaches are highly correlated, and the two analyses tend to provide similar results ( Rosenthal et al., 2012 ).

It is because of these complexities in interpreting bisexual identities that researchers have turned to more objective measures of bisexual orientations. Specifically, researchers have used measures of genital arousal to determine whether men who identify as bisexual also have bisexual arousal patterns (given that some men may identify as bisexual despite having homosexual arousal patterns).

Among women, bisexuality appears to be a more stable identity pattern, as well as a more stable pattern of self-reported sexual attraction ( L. M. Diamond, 2008 ; Savin-Williams & Ream, 2007 ). This is consistent with the fact (reviewed earlier) that women are more likely than men to report bisexual patterns of attraction. Specifically, a woman who comes out as bisexual is more likely to be “correct” in this assessment than is a man, and hence more likely to retain that label over time and to find that it provides an adequate representation of her attractions and behavior. Among both men and women, negative stereotypes about bisexuality, perpetuated within the gay community as well as in the culture at large, make it difficult to interpret individuals’ adoption or rejection of bisexual labels. Just as some individuals with homosexual patterns of attraction may identify as bisexual because they perceive it to be an easier transition to make than a direct transition to homosexual, some individuals with bisexual patterns of attraction may identify as homosexual because they are aware that some members of the gay community view bisexuals as untrustworthy, closeted, or promiscuous ( Tania & Mohr, 2004 ).

Researchers of bisexuality have often acknowledged the existence of transitional bisexuality ( Fast & Wells, 1975 ; Harwell, 1976 ; Klein, 1993 ; H. L. Ross, 1971 ; M. W. Ross, 1991 ), but it remains poorly understood. The very limited existing evidence supports the idea that at least among men, transitional bisexuality identity does not necessarily imply a history of bisexual attractions ( Guittar, 2013 ). Rather, men may adopt transitional bisexual identities in the process of trying to make sense of divergent parts of their current and previous attractions and histories, such as the fact that they may have had emotionally satisfying romantic relationships with women despite feeling sexual attractions only toward men, or the fact that their previous heterosexual encounters may have been unsatisfying but not distasteful. Others may feel that it is easier to admit one’s homosexual feelings if they are not appearing to “rule out” the possibility of heterosexual attractions and relationships. Finally, some men may initially identify as bisexual because they have bisexual patterns of attraction but, eventually, switch to a gay identity because most of their attractions and all of their sexual behaviors involve men.

Some individuals who will eventually identify as homosexual temporarily adopt a bisexual identity before doing so ( Lever, 1994 ). Transitional bisexual identification appears to be more common in men than in women. In a large U.S. national survey conducted by the LGB-focused news magazine The Advocate ( Lever, 1994 ), 40% of gay-identified men reported having previously identified as bisexual. Two recent longitudinal studies of nonheterosexual-identified youth found that changes of identity from bisexual to homosexual were particularly common among males ( Rosario, Schrimshaw, Hunter, & Braun, 2006 ; Savin-Williams, Joyner, & Rieger, 2012 ). Transitional bisexual identity and behavior may also occur among women on their way to a homosexual identity ( Lever, 1995 ), though women’s sexual identities also not infrequently change from homosexual to bisexual ( L. M. Diamond, 2008 ).

The topic of bisexuality requires special precision in discussing identity, attractions, behavior, arousal, and orientation, given the prevalence of discrepancies among these domains. Research has found that some individuals who identify as bisexual show patterns of sexual arousal (and sometimes patterns of sexual behavior) that appear to be predominately heterosexual or homosexual, whereas some individuals who identify as heterosexual or homosexual show bisexual patterns of genital arousal, attraction, or behavior. Such discrepancies reflect widespread variability in individuals’ and communities’ definitions of “bisexual,” as well as variability in individuals’ motivations for identifying as bisexual. Although it seems reasonable to presume that bisexual-identified individuals exhibit both same-sex and other-sex attractions and sexual behavior, other patterns are possible. Some individuals identify as bisexual because they have previously engaged in sexual activity or intimate relationships with both men and women, even if their current sexual attractions are exclusively toward the same sex or the other sex. Other individuals identify as bisexual because they periodically experience sexual attraction toward both sexes, even if their sexual behavior and identity is exclusively homosexual. As a result of this diversity, the population of individuals reporting bisexual attractions, behavior, or identity contains individuals with a range of different orientations.

The past decade has seen a surge of research about bisexuality. This has reflected (a) growing awareness that a nontrivial proportion of nonheterosexual people identify as bisexual; (b) the widespread acknowledgment that bisexuals have often been both socially and scientifically marginalized; (c) scientific controversy concerning bisexual orientation; and, notably, (d) appreciable research funding by the American Institute of Bisexuality ( Denizet-Lewis, 2014 ). Until recently, research on sexual orientation often ignored bisexuality, as reflected in the two most common approaches to dealing with potentially bisexual subjects: excluding them or combining them with monosexual subjects.

Causes of Sexual Orientation

The question of what causes different people to be heterosexual, bisexual, or homosexual has evoked intense interest among the general public (e.g., Finkelstein, 2006; Swidey, 2005). However, the question is unusual among interesting scientific questions because it has also provoked intense controversy that in part reflects sociopolitical and moral differences among disputants (Pitman, 2011; for examples, see the relevant sections of the websites for the National Association for the Research and Therapy of Homosexuality, http://www.narth.com/#!gay—born-that-way/cm6x, and Truth Wins Out, http://www.lgbtscience.org).

The politics of sexual-orientation causation There is a strong correlation between beliefs about the origins of sexual orientation and tolerance of nonheterosexuality, a correlation that has been stable for several decades (Lewis, 2009). Specifically, those who believe that sexual orientation is “innate,” “biological,” “immutable,” or “inborn” are especially likely to favor equal rights for homosexual and bisexual people. All four words are often used by participants in the political debate about the causes of sexual orientation, despite the fact that none of the words has a straightforward, consistent, and uniform interpretation. The association between political attitudes about homosexuality and beliefs about the causes of sexual orientation is largely misplaced (Greenberg & Bailey, 1993). It is based on faulty reasoning about causation and about the relevance of causation to moral judgments. It has intruded unhelpfully into social controversies that affect the lives of homosexual and bisexual people. Furthermore, it has had a harmful effect on the science of sexual orientation. For example, in the United States, funding for basic research on sexual orientation has been limited because of political controversy. We know of several cases in which applicants for federal U.S. grants were asked to change grant titles in order to hide their topic of study from those hostile to such research. Politically controversial topics are precisely where science-based information should be sought, when it is relevant. But it is important to think clearly about what is, and what is not, relevant. We thus preface our review of scientific evidence on the causes of sexual orientation with a conceptual critique—and we hope, a correction—of the way that people often reason about how these causes relate to social, political, and moral questions.

The question of choice Do people choose to be homosexual or heterosexual? This question is perhaps the most common causal question asked in the sociopolitical context. It is asked much less commonly in scientific contexts because, as we shall see, it is a bad question. This is partly because there are at least two different, mutually inconsistent meanings of “choice” that are often conflated. Choice as uncaused action The correlation between one’s beliefs about the causes of sexual orientation and one’s degree of tolerance of nonheterosexuality appears to be based on the following logic: If there are causes—other than free will—that lead certain people to be nonheterosexual, then those people were never entirely free to be heterosexual and hence cannot be held responsible for their nonheterosexuality. For example, finding a gene that increases the chance a man will be homosexual would mean that the man is not completely free to choose to be heterosexual. To the extent that the gene causes his homosexuality, we should neither blame him nor discriminate against him. This is the essence of the argument regarding sexual orientation and choice. Yet this is a bad argument, and the word “choice” (and associated concepts such as freedom and responsibility) lies at the root of the problem. Why would discovery of a gene for sexual orientation imply that homosexuality is not freely chosen? It would do so only if we could assume that free will is the null hypothesis on which causal studies chip away. This assumption is not scientific, however, and is not intellectually defensible (Dennett, 1984; Pinker, 2003). For instance, to the extent that a trait is not genetic, it is caused by the environment, not by free will. If a trait is not present from birth, then it is caused by events occurring after birth, not by free will. Choice as decision to act There is an alternative sense of “choice” that is more meaningful: the sense of making a decision. This ordinary-language sense of “choice” is something that is commonly understood. “I chose to raise my hand,” “I chose to eat broccoli,” “I chose to rob a bank,” and “I chose to have sex with that person” are all meaningful sentences. It is this sense of “choice” that people likely mean when they debate whether sexual orientation is a lifestyle choice. Note that cause has nothing to do with it. The four sentences all make sense even though the respective choices to which they refer could all be determined by a combination of genetic and environmental factors. People may choose to do things for environmental reasons—perhaps a woman has been offered a million dollars to raise her hand. Or they may choose them for genetic reasons—perhaps a man chose a blue car because his genetically determined color blindness made red and green cars unappealingly gray. In a deterministic world (which behavioral scientists assume), decisions have causes. The meanings of words can be illuminated by how people use them, and an important regularity in the way people use “choice” concerns the distinction between behavior and feelings. We choose our actions, but we do not choose our feelings. Consider the following two sentences: “I choose to have sex with partners of my own sex.” “I choose to desire to have sex with partners of my own sex.” The first sentence is conventional and sensible; the second sentence is neither. Einstein summarized Schopenhauer’s famous argument appropriately and thusly: “Man can do what he wills, but he cannot will what he wills” (as quoted in Planck, 1933, p. 201). Applied to sexual orientation, it makes sense to say that people choose their sexual partners, but it doesn’t make sense to say that they choose their desires. Sexual orientation is defined as relative desire for same-sex or other-sex sex partners. Thus, it makes no sense to say that one chooses one’s sexual orientation. One does, however, choose to behave consistently or inconsistently with one’s sexual orientation. That is a lifestyle choice.10 A flawed question The question of whether or not people choose their sexual orientations has clouded rather than clarified thinking. We can answer the question without knowing anything scientific about the causes of sexual orientation, because the answer depends entirely on what we mean by “choice” and “choose.” If a “choice” requires a lack of causes, then people cannot choose their sexual orientation because all human behavior has causes. If “to choose” means “to make a decision,” then we do not choose our sexual orientations because sexual orientations are patterns of sexual desire, and we do not choose our sexual desires. Of course, advocates of therapeutic attempts to change sexual orientation might argue that even if sexual desires are not initially chosen, individuals can choose to alter their desires through processes of conditioning and reinforcement. As we discuss in greater detail below, however, there is no evidence that such attempts are successful.

Is causation relevant at all? Even if the question of whether people choose their sexual orientations is a bad one, this does not necessarily mean that all questions about the causes of sexual orientation are irrelevant to political, social, and ethical concerns. Unfortunately, such relevance is usually assumed rather than argued for. Furthermore, in cases where the argument is spelled out, it typically amounts to an argument that any discernable causes negate free will, which we have seen is invalid. A more comprehensible basis upon which people differ in their moral and political responses to homosexuality is in their beliefs about its consequences rather than its causes. People who believe that homosexuality has negative effects on psychological, moral, or social functioning will disapprove of homosexuality and may seek to discourage it by restricting its expression and limiting the rights of homosexuals. In contrast, people who believe that there are no negative consequences of homosexuality will favor the rights of homosexuals to live openly and to enjoy the same rights as heterosexuals. These grounds—rather than questions of causation and choice—are the appropriate grounds on which the battle for equal rights for nonheterosexual people should be fought.11 This is not to dismiss the importance of basic research on sexual orientation, including research on causation. Despite our pessimism about causal research’s importance in resolving social questions, there are some specific limited cases in which it is relevant. One example concerns the belief, expressed by President Museveni in Uganda as well as anti-homosexual activists worldwide, that homosexual people attempt to recruit others into homosexuality. Equally important is the question of whether individuals’ sexual orientation can be changed through “conversion,” “reparative,” or “reorientation” therapies and whether such therapies should be deemed safe and effective by psychological organizations. Claims about both homosexual recruitment and the success of conversion therapies are causal claims, and we examine them in a later section. In our view, the single best justification for studying the causes of sexual orientation is scientific, not sociopolitical. Quite simply, sexual orientation is a basic human trait that influences identity and behavior at both the individual and the group level, and hence it is fundamentally important and interesting to understand its causes and development. Toward this end, in the following sections, we summarize the current state of this understanding.

A taxonomy of causal questions Non-experts are exposed to scientific findings primarily through the news media. Journalists try to make a judicious compromise between scientific accuracy and simple comprehensibility; sometimes they fail. Sometimes even scientists are not as precise and accurate as they should be. A plethora of terms have been used as alternatives in the two sides of the nature-nurture debate. For nature: “biological,” “genetic,” “hereditary,” “heritable,” “innate,” “inborn,” “natural,” and “essential.” For nurture: “chosen,” “learned,” “acquired,” “environmental,” “socialized,” “unnatural,” and “socially constructed.” Some of these terms have originated from scientists and have been accurately applied to particular studies but overgeneralized to other, inapplicable studies. Other words, such as “chosen,” are rarely used by scientists but commonly (and erroneously) used by laypersons. Importantly, words that are treated as synonyms in informal usage often have quite different formal definitions. Thus, it is important to correct some common misunderstandings about causal terminology before proceeding. The first two columns of Table 2 list some words most commonly used in the nature-versus-nurture debate. Adjacent words in the same “Nature” or “Nurture” columns represent synonyms—conceptual distinctions that are dissociable but not completely independent. Words in the same row but different columns represent conceptual opposites (or complements). The final two columns of Table 2 convey our judgments of whether the conceptual distinctions implied by a given word are valid and whether those distinctions are precise or require further elaboration. Table 2. Causality-Related Terms Commonly Used in Scientific and Political Debates About Sexual Orientation View larger version We have already addressed the conceptual vacuity of “choice.” Another word that obscures more than it clarifies is “biological.” Because all behavior requires participation of the brain and body, and such participation is at least in principle measurable, all behavior is “biological” (Greenberg & Bailey, 1993). Thus, the word denotes nothing unique about any trait or behavior. Although “biological” is often used synonymously with other words in the “Nature” column of Table 2, that practice should stop, and more accurate and precise words should be used instead. The extent to which a trait is “genetic” or “environmental” is a conceptually meaningful and precise question, although it may often be practically difficult to provide a precise answer. The extent to which people differ in a trait as a result of genetic or environmental differences among them can be estimated as heritability, which is expressed as a proportion ranging from 0 (only environmental differences matter) to 1 (only genetic differences matter), using methodology including twin and adoption studies. The idea that hormonal differences may lead to sexual-orientation differences has been especially influential (Ellis & Ames, 1987; LeVay, 2011). The extent to which trait differences reflect differences in hormones is generally meaningful and entirely separate from the question of genetic influence. If genes for sexual orientation exist, they may or may not operate via hormonal pathways. Conversely, hormonal differences may not depend on genetic differences (aside from the initial role of the Y chromosome in triggering androgens in males). Thus, evidence regarding heritability provides no evidence regarding hormonal influence, and vice versa. The extent to which a trait is hormonally influenced is a less precise question compared with the extent to which it is genetically influenced. A trait might be hormonally influenced via levels of hormones currently present in relevant tissue; alternatively, it may have been influenced via levels of hormones present during the organization of the tissue, or it may be influenced by receptivity to hormones, either at present or during organization; there are likely other possibilities. Nor do assessments of hormone levels yield a numeric index comparable to heritability. Innateness is a famously difficult concept (Elman et al., 1998), in large part because it has a number of different connotations (Mameli & Bateson, 2006). For sexual orientation, the question of innateness is most meaningfully asked as follows: When people have different sexual orientations, is this because they had different postnatal social experiences, or did the different sexual orientations emerge despite their social experiences? If differences in social experiences matter, then sexual orientation is not entirely innate; if they do not matter, then it is.12 For humans, “social experiences” mean experiences involving other people, and the mechanisms through which such experiences are thought to influence development generally comprise diverse forms of learning and reinforcement. In contrast, mechanisms of innate development include practically anything else: genes, hormones, random prenatal developmental perturbations, infections, and other factors too numerous to list. Thus, although genetic and hormonal influences on sexual orientation constitute innate influences, not all innate influences require either genes or hormones. Arguably, the innateness debate is best focused on whether particular candidate social influences matter rather than whether any social influence matters. The question of whether sexual orientation is “natural” or “unnatural” has tended to be asked in the context of religious debates, with anti-homosexual advocates insisting that homosexuality is unnatural (e.g., TFP Committee on American Issues, 2004) and their opponents insisting it is natural (e.g., A. Jackson, 2013). What can this question mean? It has at least three different interpretations. First, does homosexuality occur in nonhuman animals? Second, is homosexuality a result of human evolution? Third, is homosexuality consistent with natural law? The third question is philosophical rather than scientific (Pickett, 2011), and we do not consider it here. In contrast, the first two interpretations of the question are scientifically meaningful and interesting. Answers to both are of no moral consequence, however. The final row of Table 2 refers to a debate that is neither conducted mainly by scientists nor by members of the general public. Rather, whether sexual orientation is an essential or a socially constructed categorization of people has largely been the preoccupation of scholars of human culture and society, including philosophers, historians, social theorists, cultural anthropologists, and sociologists (e.g., Norton, 1997; Stein, 1992). One of the main issues in this debate concerns the degree to which sexual orientation manifests, and is recognized, in a similar manner across cultures, both geographically and historically. Those who believe that sexual orientation is socially constructed emphasize cultural variation, whereas those who believe that it is an essential human trait are impressed with its cross-cultural regularities. We address the cross-cultural evidence in the following section.