New study helps explain ‘Darwinian paradox’ of same-sex attraction

Men carrying genetic factors linked to bisexual attraction tend to have more children, a new study has found.

The paper published Wednesday in Science Advances found evidence for genetic variations that contribute to both bisexual attraction in men and a heightened appetite for risk.

Those findings help resolve a paradox at the heart of evolutionary theory — while also raising difficult new questions about the genetic roots of same-sex attraction.

For generations, evolutionary biologists have seen homosexual or same-sex attraction as a hole in evolutionary theory, as it is an at least somewhat inherited trait that also leads those who have it to have fewer children.

“So if you put these two things together, it doesn’t make sense,” lead author Jianzhi Zhang of the University of Michigan told The Hill. 

“Those genes lead to fewer children, so that means they are being selected against in the population by natural selection. So gradually, they should just disappear from the population. So why do they still exist?”

That’s a question that goes far beyond the world of human sexuality. Same-sex attraction and behavior are widespread across the animal kingdom, from male gentoo penguins co-raising eggs, sex among all-male bachelor packs of gorillas and “seasonally bisexual” flying fox bats. 

“It’s just wherever you look. I can give you papers on beetles, spiders, flies, fish, flamingos, geese, bison, deer, gibbons, bats,” said ecology and evolution researcher Jackson Clive in an interview with Imperial College London. 

“Loads of bats, bats get up to all sorts,” Clive added. “The list is endless.”

That’s an exaggeration: In fact, the list is simply very long, including more than 1,500 species, according to a Nature Ecology and Evolution study.

Theories for a resolution to the paradox are widespread — and include arguments that there is no paradox at all. In the Nature Ecology and Evolution study, for example, the authors argue that scientists’ assumption that opposite-sex attraction is normal and ancestral “has not been rigorously examined.”

Instead, that team hypothesized “an ancestral condition of indiscriminate sexual behaviors directed towards all sexes.”

In an essay in Scientific American, the Nature authors argued that the foundations of animal sex may have been laid down long before the relatively clear physical differences between males and females that the idea of the paradox takes for granted.

“It is unlikely that the other traits required to recognize a compatible mate — differences in size, shape, color or odor, for example — evolved at exactly the same time as sexual behaviors,” they wrote.

“Indeed, indiscriminate mating can be more beneficial than it is costly.”

Another 2023 study, also in Nature Ecology and Evolution, found that male macaque monkeys who mounted each other seemed to strengthen their relationship in a way that “mediated coalitionary partnerships that have been linked to improved reproductive success.”

Or as a separate Scientific American piece about three “lesbian” capuchin monkeys in a Los Angeles sanctuary put it in September, “it’s clear being a little gay nearly every day helps primates get their way — in pleasure and in life.”

But others suggest the resolution to the paradox is in a concept called “pleiotropy,” in which the collection of genes that leads to one trait also leads to another.

Many scientists have posited that the fact that same-sex attraction can be inherited makes it an “antagonistic pleiotropy” — a shared gene that leads to different behaviors that work against each other to increase and decrease the number of offspring.

In that hypothesis, a gene that led to bisexual attraction — and therefore having fewer children — could still be preserved if it was also tied to another trait that led to having more children.

For example, scientists have posited that genes for same-sex attraction might lead to more coparenting — or a caste of non-child-having, same-sex uncles and aunts — or that male homosexuality might be a spillover of genetic traits that made heterosexual females more fertile.

But these hypotheses shared a common problem, Zhang said: “Most of them do not have empirical evidence.”

In 2021, however, researchers published a study in Nature Human Behavior that did offer empirical evidence for the idea that same-sex attraction might be linked to other traits that increase the number of children a person has.

By examining the U.K. Biobank — a massive database of 450,000 genetic samples associated with detailed behavioral surveys — researchers found a host of genetic variants that correlated with same-sex behavior.

Then, by comparing those variants to the reported number of sexual partners of either gender the person carrying them had, they found that the genes “associated with ever having had a same sex partner were also associated — among people who had never had a same-sex partner — with having had more opposite-sex partners,” as co-author Brendan Zietsch explained in an essay in The Conversation.

While the mechanism for how this worked was unclear, Zietsch’s team speculated that these factors might work together to also “make someone ‘more attractive’ in broad terms.”

In the study published Wednesday, Zhang and lead author Siliang Song went searching in the U.K. Biobank data for a more precise mechanism.

Their first discovery came when they sought to see if they could break up the category of bisexual behavior from strictly same-sex behavior.

“Because that’s a controversy — is sexuality discrete or continuous? Do they have [a] different genetic basis or do they have the same genetic basis?” Zhang said.

Answering that question revealed something unexpected: evidence of different genes associated with bisexual and exclusively same-sex behavior — with the genes linked to bisexual behavior also being related to having more children, while those linked to strictly same-sex behavior were not.

This pointed to a partial resolution of the paradox surrounding bisexual behavior — while also seemingly reaffirming the paradox when it came to strictly homosexual behavior.

Then, Song and Zhang looked for other traits from the Biobank survey responses that correlated with bisexual attraction.

They found that a heightened appetite for risk-taking was associated with genetic factors connected to both bisexual attraction and having more children — a link so strong that, when they controlled for it, the apparent connection between bisexual attraction and having more kids disappeared. 

“People who carry bisexual genes have more children. And the reason they have more children is because the so-called the bisexual genes [mean that they] are willing to take more risks,” Zhang said. 

And for some people, “more risks” will mean more sexual partners, Zhang added. “Then they will have more children. That’s what the results suggest.”

However, whatever link exists between the genetic factors associated with bisexual orientation, risk and having more children, past reporting by Zhang and Song suggests its effect is now largely obsolete due to contraception. 

In a study the pair published last year in the Proceedings of the National Academy of Sciences (PNAS), they dug into a key element of Zietsch’s findings — that people carrying genes linked to same-sex attraction had more children.

“But it’s very difficult for that to work in modern society, because having more sexual partners does not necessarily mean more children, given that we have contraception,” Zhang said.

When they examined the Biobank, Zhang and Song found that before the 1960s — when oral contraceptives, or “the pill” became widespread in England — more sex really did lead to more children, and with it, more spread of the theoretical genetic basis for same-sex attraction.

“But after the 1960s, this relationship disappeared,” Zhang said. The data, he said, suggested that if a linked ancestral trait had led to both same-sex attraction and having more children, that link hadn’t survived universal access to contraception, at least in the United Kingdom.

The widespread availability of the pill, Zhang and Song wrote in PNAS, “could abolish the aforementioned mechanism for the genetic maintenance of [same-sex behavior].”

Zhang noted that there’s now a countervailing force against contraception’s effect: the historically novel ability for same-sex couples to have their own children. 

But he argued that isn’t a big factor: Strictly homosexual people have 75 percent fewer kids on average than strictly heterosexual people, while bisexual people have about 30 percent fewer, he said.

“We predict that for exclusive same-sex behavior, they will decline in frequency over time in the future,” he said. “But because same-sex behavior is influenced more by the environment than by genes, whether the proportion of people in this society will increase or decrease is unknown.”

He also acknowledged that these numbers rely on something very hard to prove: that the British survey respondents were telling the truth about — and are fully cognizant of — their own same-sex attractions, as opposed to their actions.

Surveys, he noted, are notoriously unreliable. 

That reliability problem disappears if you assume survey errors are random — that as many bisexual people will “misreport” as homosexual, or heterosexual people as bisexual, he said. 

But if the errors aren’t random — if, for example, somewhat same-sex oriented people are more likely to misidentify themselves as heterosexual — then bisexual males might have the same number of children as strict heterosexuals, if not more.

If so, that would undercut the idea of a Darwinian paradox, suggesting a more straightforward link between same-sex attraction and having more children. 

Zhang also pointed to a more obvious issue with the study: that the data set is based in the United Kingdom, among largely European participants.

 “So we don’t know whether our results apply to other populations,” Zhang said.