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One is the loneliest number, says the old pop tune. When it comes to , most people will agree that the partnered version is better than the solitary one. But how many partners are we supposed to have?

It’s a curious question, at once personal—you won’t get it at a job interview—and political, in that the answer may signify a person’s values and social standing. It’s also a season-of-life question: It seems crucial in your 20s; but in your 70s, who cares or can remember?

Our interest in number of sexual partners has much to do with social comparison. We live in groups, and our place in the group matters. One way by which we judge ourselves, our worth, our standing, and our progress—one way, in other words, by which we know ourselves—is in comparison to others. A number only has meaning in relation to other numbers.

Social scientists have their own reasons for studying the issue. First, it can serve as a gauge of changing social mores and attitudes about . Second, number of partners may predict other important outcomes, such as higher STI risk or reduced prostate cancer risk.

Third, if we find out what factors influence the number of partners people end up having, we may gain some control over this behavior. All societies busy themselves with the control of sexual activity and for understandable reasons: patterns of sexual activity influence important social and economic issues such as population growth, public , and child rearing practices.

Human behavior as a whole is multi-determined, thus we are unlikely to find a sole cause shaping people’s sexual choices. Behavior is also contextual. The meaning of, “I’ve slept with 10 people so far” to the individual, to society, and to scientific prediction will change depending on the speaker’s age, , , , , occupation, nationality, and historical period.

That said, in the past 50 years we have learned a few interesting things about the causes and consequences of a person’s number of sexual partners.

It is clear that play a role in the number-of-partners game. Genes may indirectly influence sexual behavior through traits ( like more people, and more people like them, sexually and otherwise) or physical appearance (on average, very short men have fewer sexual partners than taller men).

Evidence suggests that the tendency to seek more or less sexual variety is genetically coded. Recent research has found a link between in-utero exposure (gauged reliably by measuring the index finger to ring finger ratio in both men and women) and preferences. The shorter the index finger in relation to the ring finger, the higher in-utero exposure, and the more likely the tendency to seek multiple sex partners.

The effects of genes often go unmentioned in the popular discussion (and policy debates) surrounding sexual behavior. This is in part because people (and policy makers) often erroneously assume that not much can be done about genetic differences. In addition, genetic effects are often obscured from easy view by a phenomenon known as ‘gene- correlation.’ Scientists have identified three types of gene-environment correlation.

Passive gene-environment correlation has to do with the fact that usually supply both the child’s genes and early environment. Therefore, child outcome that looks like the result of environmental influence may in fact be the result of heredity. For example, the child of a reckless, man is more likely to be raised in a chaotic home environment. If the child grows up to be a reckless risk-taker, was it due to his early environment, or to the genes inherited from the father? It's hard to know.

Evocative gene-environment correlation has to do with how some genetic traits elicit certain reactions from . For example, an early-maturing girl may experience teasing in school, which results in low due to her physical appearance. If the same genes that predisposed her toward early maturation also predispose her toward earlier sexual activity, there will be a correlation between and age of first sex, not because low self esteem causes early sex, but because a certain genetic constellation brings about both.

Active gene-environment correlation has to do with the fact that a person’s genetic endowment disposes them to choose certain environments over others. seek out social connections, including sexual ones, and may find reading boring. Less time at the library (an environmental experience) will in this case correlate with promiscuity, but it is not the cause of promiscuity.

The bottom line result of gene-environment correlation is that we often confuse environmental predictors with environmental causes.

For example, research has long documented a link between early sex initiation and later promiscuity for both males and females. This link is often interpreted to mean that early initiation causes later sexual . However, twin research has shown that identical ’ different age of sexual initiation did not result in differences in sexual risk-taking. It appears that early initiation and later promiscuity, rather than being cause and effect, are both strongly influenced by genes.

To be sure, genetic influence is not the only game in town. In fact genetic influence is often dependent on the surrounding environment. The onset of , while largely under genetic control, is nonetheless subject to environmental influences such as nutrition and early family environment.

Moreover, genetic influences may change over time. There is little difference in the ages of sexual initiation in identical twins when it happens early (ages 13-15). However, when it happens later, there is much more difference.

The effects of genes may also be overridden by the influence of environmental forces. A study of Australian twins found that a history of overrode the effects of in predicting sexual initiation in twin girls.

In fact, with due respect to genes, social and cultural forces are always powerfully at work in shaping behavior, sexual or otherwise. The fact that the average number of sexual partners has increased substantially between the Greatest Generation (2.16) and the Baby Boomers (11.68) is unlikely due to genetic differences between those populations; cohort-specific environmental conditions are the more likely culprits. For example, the Greatest Generation came of age before the advance of the birth control pill; Baby Boomers came of age after the pill (and before AIDS).

As with all matters sexual, is bound to play a role. Traditionally, men are known to report average numbers of partners up to four times higher than women’s. Alas, people are also known to lie about this, and due to the notorious ‘double standard’ (by which men’s stock tends to rise and women’s tends to fall with increased number of partners), women and men may lie in opposite directions.

In addition to straight-up liars, the gender disparity in number of partners may also be linked to outliers. A few men report very large numbers of partners, thus skewing the male average up, just like a few billionaires in a room full of middle class people would skew the average income upward.

Moreover, reported differences may be the result of different estimation methods. A study by Brown and Sinclair (1999) found that regardless of gender, those who estimate by “enumeration” (counting all the names they remembered) report systematically lower numbers than those who use “rough approximation” (just winging it). As it happens, women tend to favor the former method while men favor the latter.

Finally, when the median (the number that cuts a distribution in half) is calculated rather than the average, the differences between the genders appear to diminish. For example, CDC data from 2011-2013 shows the median number of opposite-sex partners in lifetime among men and women aged 25-44 years of age was 6.7 and 4.3 respectively.

Still, the partner-number gap remains a robust finding in the literature, and with due respect for socio-cultural influences, there is good evidence that it is in part evolutionary, shaped by differences in short term mating strategies.

Evolutionarily speaking, short-term can be beneficial for both sexes, albeit in different ways. Men may further ‘spread their seed’ in short-term sexual relationships and increase the odds of an offspring. Women may be able to get high quality sperm from a healthy male who—given his and increased mating options—is unlikely to stick around for the long haul. Consequently, males and females differ in how they pursue short term mating strategies.

Casual sex is less risky for a man, both physically (men on average are physically stronger) and biologically (few men die in childbirth). Consequently, all over the world men are quicker to say yes to sex than women. Maximizing the number of partners offers a reproductive advantage for men—multiple concurrent partners may lead to multiple concurrent offspring. Not so for women, who can only have one offspring at a time, regardless of the number of concurrent partners. Thus, all around the world, men who opt for short-term mating pursue quantity—large numbers of partners. Women who seek short-term mating pursue quality—they look for masculine men high in social dominance.

Still, while our underlying biological wiring is designed to pursue the biological imperative (move the genes forward), our overlaying socio-psychological systems have evolved to deal with multiple, non-reproductive problems of living (such as boredom) and pursue proximal non-reproductive (such as feeling happy).

Most people having sex right now around the world (roughly 166,666 people by some estimates) are not pursuing reproduction, but are more likely looking for the increased sense of wellbeing provided by , touch, connection, and .

Is number of partners related to ? I have seen no research suggesting that more partners beget more . Does promiscuity augurs ill? Not necessarily. A longitudinal study that followed 1000 children in New Zealand into their 30s found no significant relationship between number of partners and mental health problems such as or (higher number of partners did predict substance risk for women).

Blanchflower and Oswald (2004), analyzing a random sample of 16,000 adult Americans, found that, “The happiness-maximizing number of sexual partners in the previous year is calculated to be 1.” Cheng and Smyth recently followed up with a similar investigation of 3,800 adult Chinese. The results: “We find that the happiness maximizing number of sexual partners is one.”

One, it turns out, may in fact be the least number…