The search for the “gay gene” is rooted in a fraught question: Does homosexuality have a biological basis? The reasons society asks these questions are complex. The answers scientists have uncovered are somewhat less so. As a new PNAS paper published Monday shows, there is evidence that certain biological processes are linked with male sexual orientation.

"Whether same-sex sexual orientation runs in families is relevant to the idea that sexual orientation might be influenced by genetics in some way."

The team, which included University of Toronto Mississauga assistant psychology professor Doug P. VanderLaan, Ph.D., used data on 827 men to link three “biomarkers” to male sexual orientation: fraternal birth order, handedness, and “familiality.” Previous studies had shown a link between each of these characteristics and sexual orientation; what VanderLaan and his colleagues did was investigate how these biomarkers overlapped in gay men.

They looked at the distribution of these biomarkers in their sample of men, finding that while most men fell into a category of “no biomarker,” the rest could be divided into three subgroups, each based on one of the biomarkers. And within each of those subgroups, there was a higher proportion of non-heterosexual men.

“The respective literatures on how familiality, fraternal birth order, and handedness relate to sexual orientation each go back several decades,” VanderLaan, the study’s corresponding author, tells Inverse. “Whether same-sex sexual orientation runs in families is relevant to the idea that sexual orientation might be influenced by genetics in some way.”

The Significance of “Biomarkers”

Participants were recruited via Facebook during Toronto Pride in June 2015. Flickr / Andrew Paterson

Biomarkers are evidence of underlying biological processes that scientists can easily measure. They’re an indirect way of interrogating how a person’s biology influences other characteristics, like their sexuality.

For example, familiality — in this case, the sexual orientation of other men in a person’s family — is a proxy measurement for genes, allowing scientists to study the heritability of sexual orientation. “Research on familiality and genetic linkage of sexual orientation started to gain momentum in the early 1990s,” VanderLaan says. Previous studies have shown that same-sex sexual orientation tends to cluster in families, on both mom’s and dad’s side.

Birth order, meanwhile, is a proxy for a fascinating theory called the “maternal immune hypothesis.” Researchers showed before the ‘90s that gay men tended to be born later among their siblings, but it wasn’t until the early to mid-‘90s that they realized it was later birth order among brothers in particular, explains VanderLaan. This led to the maternal immune hypothesis, which suggests that, as a mother births more boys, her immune system builds up a response to male-specific antigens, and this response eventually influences the sexual differentiation of later sons.

Then there’s handedness, which has been linked to a number of traits including sexual orientation because it’s linked to right-left brain lateralization, which is thought to occur prior to birth and shows sex differences, says VanderLaan (males are more likely to be left-handed or ambidextrous). This hints at the idea that sexual orientation is determined to some degree before a person is born, although VanderLaan admits that scientists don’t fully understand how handedness is decided in the body.

How Biomarkers Relate to Sexual Orientation

Non-righthandedness in men has been linked to non-heterosexual sexual orientation in previous research. Unsplash / Alejandro Escamilla

The team’s analysis of these traits in their participants, all of whom were recruited via Facebook ads during the June 2015 Toronto Pride Festival, showed that “the three profiles associated with a biomarker were composed primarily of nonheterosexual men,” the authors write. In other words, the majority of the individuals in the groups defined by having more older brothers, left-handedness, and a high proportion of gay/bisexual relatives were nonheterosexual. It bears repeating that the majority of both heterosexual and non-heterosexual men fell into the category of “no biomarker.”

All the participants had answered questions about how they self-identified and whether they were attracted and had sexual experiences with males or females in the last 12 months.

The team followed up by looking at the relationship between biomarkers and personality traits, as measured by tests like the well-established “Recalled Childhood Gender Identity/Gender Role Questionnaire” and the “Bem Sex-Role Inventory,” a measure of how a person identifies with traditionally masculine and feminine qualities.

The men in the “fraternal birth order” subgroup — those with more older brothers — were the most “female-typical” and “agreeable,” the team notes. As expected, the people in the no-biomarker subgroup were the most gender-conforming.

What This Means for Sexual Orientation

To VanderLaan, the results show that “there appear to be distinct biological processes that influence male sexual orientation development, and each of these processes seem to apply to a particular subgroup of men,” he says. “It may also be the case that these processes have different impacts on other aspects of psychological development such as gender role expression and personality.”

"…there appear to be distinct biological processes that influence male sexual orientation development, and each of these processes seem to apply to a particular subgroup of men."

Asked whether he was surprised by the findings, VanderLaan says, “In some ways, no. But in other ways, yes.” The findings not only support previous studies showing the role of those three biomarkers; they also elaborate on how “clearly” they “applied to distinct subgroups of men,” he continues. “And, also, how these subgroups showed different patterns in terms of gender-role expression and personality traits like agreeableness.”

He’s careful to point out that there’s a lot we don’t know about how sexual orientation develops, especially in light of the fact that the three biomarkers didn’t apply to a large proportion of non-heterosexual men in the study (and the majority of the participants in general). It’s possible, even likely, that there are many more developmental pathways at play.

For now, says VanderLaan, we can safely take away the idea that “there appear to be distinct biological processes that influence male sexual orientation development, and each of these processes seem to apply to a particular subgroup of men.” These processes, he continues, may also affect gender role expression and personality.

Other scientists who research biology’s influence on sexual orientation have taken care to emphasize that sexual orientation cannot and should not be boiled down to a single characteristic. In a 2018 interview with Inverse regarding the interplay between finger length and female sexual preference, Michigan State University neuroscientist S. Marc Breedlove, Ph.D., noted: “If we look at digit ratios in a random sample of people, where some 95 percent will be straight, then using digit ratios to detect ‘hidden lesbians’ would be very ineffective.”

Likewise, a person’s brothers, family, and writing hand may offer insights into the way their sexual orientation developed, but they don’t make it possible to point out who’s gay or not. Not that anyone has any business doing that, anyway.

No matter how our biology might influence sexual orientation, we can’t ignore the roles that culture and society play in a person’s sexual self-expression. Our lives are shaped but not determined by our genes, and non-biological factors may very well override a person’s biology. To what degree, we don’t know; but what we do know is that sexuality is primarily a matter of self-identification, whether or not that’s consistent with what our genes dictate.

Abstract:

Several biological mechanisms have been proposed to influence male sexual orientation, but the extent to which these mechanisms cooccur is unclear. Putative markers of biological processes are often used to evaluate the biological basis of male sexual orientation, including fraternal birth order, handedness, and familiality of same-sex sexual orientation; these biomarkers are proxies for immunological, endocrine, and genetic mechanisms. Here, we used latent profile analysis (LPA) to assess whether these biomarkers cluster within the same individuals or are present in different subgroups of nonheterosexual men. LPA defined four profiles of men based on these biomarkers: 1) A subgroup who did not have these biomarkers, 2) fraternal birth order, 3) handedness, and 4) familiality. While the majority of both heterosexual and nonheterosexual men were grouped in the profile that did not have any biomarker, the three profiles associated with a biomarker were composed primarily of nonheterosexual men. We then evaluated whether these subgroups differed on measures of gender nonconformity and personality that reliably show male sexual orientation differences. The subgroup without biomarkers was the most gender-conforming whereas the fraternal birth order subgroup was the most female-typical and agreeable, compared with the other profiles. Together, these findings suggest there are multiple distinct biodevelopmental pathways influencing same-sex sexual orientation in men.