You don’t have to be a genius to understand sex. Even the biological equations you find in textbooks are simple: X + X = ♀ ; and X + Y = ♂. But as science investigates more closely, it becomes clear that a pair of chromosomes doesn’t always suffice to define human sexuality.
In the cultural realm, a new perspective on sex and gender is already widely embraced. “Nonbinary” definitions of gender – transfeminine, genderqueer, hijra – have made their way into the vernacular.
What is less apparent are the changes taking place in the biological sciences that appear to support this cultural movement. Here, the emerging picture is that complex gene networks are involved in determining the “femininity” or “masculinity” of a human being.
This inevitably brings to the fore the polarizing question of whether people are born gay or become so. Does a genetic predisposition determine your gender identity, or is this something decided later on in life?
This contentious question, in turn, suggests a long-standing paradox. Between two and five percent of men worldwide are gay. Gay men have fewer children. Given those two facts, one would expect the trait to disappear over time. But it has not. Why?
Recently, a team of researchers carried out the largest-ever genetic study of sexual orientation and found data that supports one possible explanation.
The same genetic factors that predispose people to homosexuality may also – when heterosexuals have them – lead to more sexual partners. This means higher mating success!
A New Science of Sex and Gender
What the team found was that DNA signals linked to gay sexual experiences also often appeared in straight men who had multiple sex partners. The team also noted that straight men with the gay-linked variants were, on average, more “physically attractive” than others.
Such trade-offs are a fact of evolution, the researchers explain. For instance, gene variants that can cause sickle-cell anemia also lend protection against malaria.
The resulting balance means the sickle-cell gene doesn’t die out over time. The researchers say their new findings on non-heterosexual behavior, though yet inconclusive, are consistent with such a Darwinian balancing act.
The researchers have published their findings in the journal Science. But even before its publication, the study had generated debate and concern.
Several scientists who are part of the L.G.B.T.Q. community said they were worried the findings could encourage discrimination against gay people.
“This is not saying that someone is going to be heterosexual or not – it’s really saying there is going to be a slightly higher or a slightly lower chance,” says lead researcher Benjamin Neale, a geneticist at the Broad Institute in Cambridge, Massachusetts, who happens to be gay.
The team’s investigation into sexual orientation genes forms part of a larger research effort to shed light on how genes shape both disease and behavior.
Because the work could be controversial, the team chose to post their research plan online in 2017. They described their intent to perform a genome-wide association study.
Scientists first developed the genome association technique to locate genetic susceptibilities to diseases like macular degeneration and diabetes.
Instead of scouring for genetic associations in people’s illnesses, however, the team would carry out a statistical analysis of genomes and sexual behavior.
The Search Unfolds
The search unfolded along two fronts. First, the team used DNA data on more than 300,000 heterosexuals who had disclosed in a survey how many sex partners they’d had.
Then, to find genes linked to what the researchers call “non-heterosexual behavior,” the team also identified about 28,000 people who’ve had gay sexual experiences.
Many scientific institutions and private companies expressed support for the effort. This explains how the research team gained access to massive databases. These include those of the British government-funded UK Biobank and the consumer gene testing company, 23andMe.
“With these large sample sizes, we are finally discovering things we can actually kind of count on being true,” says Michael Bailey, a psychologist at Northwestern University who studies sexuality.
Scientists had already begun using this mass of data to successfully study the genetic basis for a startling range of human behavior patterns. Their findings have explained – at least in part – smoking, insomnia, intelligence, marijuana use, and even the time we spend watching television.
Inevitably, the research community began to investigate sensitive issues such as sexual orientation. Here, the researchers have found that the genetic process involved extends far beyond a specific moment when gonads form on a fetus.
Hybrids Along a Continuum
To varying extents, it seems many of us are biological hybrids on a male-female continuum. Evidence of this continuum surfaces from time to time, and might even land in the papers.
Scientists have long known that being gay – like all other aspects of human behavior – is also partly genetic. Earlier attempts to identify specific genes involved were largely unsuccessful.
But that was because there wasn’t enough genetic data available. The new study is about 10 times larger than any previous effort.
The researchers located four positions in men’s genomes that were statistically associated with their ever having had gay sex. Some 40 of them correlated with whether heterosexuals had more or fewer sex partners.
Jeffrey Reid, who is head of genome informatics at Regeneron Pharmaceuticals, says he is concerned about how such discoveries are discussed in the press. He says he worries that biased news reports could have a vast impact on already vulnerable people.
“Supposedly ‘clear evidence of a genetic basis for homosexuality may lead a parent to deem their gay son irrevocably broken and eject him from their life,” says Reid. “Alternatively, maybe some evidence of a genetic basis of homosexuality may lead a parent to embrace their child as God made them, or lead someone struggling out of darkness and into self-acceptance.”
But Neale says that the findings of the study do not explain who is gay. That’s because it relies on people’s self-reported sexual history. This means the team may have categorized people willing to experiment sexually along with those who consider themselves gay.
“Part of Who We Are”
Harvard researcher Rob Wedow says the team also had less success finding genetic links among women who’d had sex with women.
That could mean the team needs a still larger number of volunteers, says Wedow. But he admits, too, that it could reflect the shortcomings of the study’s design. It may well be that their research was unable to capture some other key nuances.
The researchers nevertheless went ahead and used the results to address the question of why homosexuality is relatively common.
The team is well-aware that the findings have far-reaching implications beyond just updating biology textbooks.
The new science of sex and gender holds the prospect of helping shape public perception and policymaking to acknowledge science-based facts.
Thus, the team’s findings may have a particular bearing on issues of personal identity, health, and the economic well-being of gay men.
They likewise open new avenues for research into the workings of female sexual desire. What is it in the gay genome sequence that makes straight men attractive to heterosexual women?
Neale hopes the research will help to settle arguments over the biological differences between genders. He says the controversy over the subject has persisted long past the time it should’ve been put to rest.
“I hope that the science can be used to educate people a little bit more about how natural and normal same-sex behavior is,” Neale says. “It’s written into our genes and it’s part of our environment. This is part of our species and it’s part of who we are.”