by Men who carry genetic factors associated with bisexual attraction tend to have more children, a new study has found.
The paper published Wednesday in Science Advances found evidence of genetic variations that contribute to bisexual attraction in men and an increased risk appetite.
These findings help resolve a paradox at the heart of evolutionary theory – while raising difficult new questions about the genetic roots of same-sex attraction.
For the past generation, evolutionary biologists have seen homosexual or same-sex attraction as a hole in evolutionary theory, since it is at least an inherited trait that predisposes those who have it to have fewer children.
“So if you put these two things together, it makes no sense,” lead author Jianzhi Zhang of the University of Michigan told The Hill.
“These genes result in fewer children, which means they are being selected against in the population by natural selection. So gradually, they should just disappear from the population. So why are they still there?”
That’s a question that goes far beyond the world of human sexuality. Same-sex attraction and behavior is widespread throughout the animal kingdom, from male penguins balancing eggs, sex among all-male gorilla packs and “seasonally bisexual” flying fox bats.
“It’s just everywhere you look. I can give you papers on beetles, spiders, flies, fish, flamingos, geese, bison, deer, hyenas, bats,” said ecology and evolution researcher Jackson Clive in an interview with Imperial College London.
“Loads of bats, bats of all sorts,” Clive added. “The list is endless.”
That’s an exaggeration: In fact, the list is very long, including more than 1,500 species, according to the Nature Ecology and Evolution study.
Theories for resolving the paradox are widespread — and include arguments that there is no paradox. In the study of Ecology and Evolution of Nature, for example, the authors argue that the assumption of scientists that the attraction of the opposite sex is normal and ancestral has not been rigorously examined.
Instead, that team hypothesized “an ancestral condition for indiscriminate sexual behaviors directed at all genders.”
In an essay in Scientific American, the authors of Nature argued that the origin of animal sex may have been established long before the relatively clear physical differences between males and females that do not involve the idea of a paradox.
“It is unlikely that the other characteristics needed to identify a compatible mate—differences in size, shape, color, or scent, for example—emerged directly at the same time as sexual behavior,” they wrote.
“Indeed, indiscriminate mating can be more beneficial than costly.”
Another 2023 study, also in Nature Ecology and Evolution, found that male macaque monkeys that were attached to each other appeared to strengthen their relationship in a way that “formed coalitional partnerships that were 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 that being a little gay almost every day helps primates get their way — in pleasure and in life.”
But others suggest that the solution to the paradox lies in a concept called “pleiotropy,” in which a collection of genes that lead to one trait also leads to another.
Many scientists have argued that the fact that same-sex attraction can be inherited makes it “opposite pleiotropy” — a shared gene that results in different behaviors that work against each other to increase and decrease the number of offspring .
In that hypothesis, a gene that encouraged bisexual attraction — and therefore having fewer children — could be preserved if it was also linked to another trait that resulted in more children.
For example, scientists have said that same-sex attraction genes — or a caste of same-sex uncles and aunts that children don’t have — could lead to more co-parenting — or that male homosexuality could be competition on genetic traits that made heterosexual. more fertile females.
But these hypotheses had a common problem, Zhang said: “Most of them lack empirical evidence.”
However, in 2021, researchers published a study in Nature Human Behavior that provided empirical evidence that same-sex attraction may be linked to other traits that increase the number of children a person has.
By examining UK Biobank – a huge database of 450,000 genetic samples linked to detailed behavioral surveys – researchers found many genetic variants associated with same-sex behaviour.
Then, by comparing those variants with the reported number of sexual partners of either gender of the person carrying them, they found that the “partner-related” genes were also involved having never had a same-sex partner — among people who have never had such a partner — and having more partners of the opposite sex,” as co-author Brendan Zietsch explained in an essay in The Conversation.
Although the mechanism of how this worked was unclear, Zietsch’s team speculated that these factors may work together to “make someone more attractive in broad terms as well.”
In the study published on Wednesday, Zhang and lead author Siliang Song searched UK Biobank data for a more precise mechanism.
Their first discovery came when they looked to see if they could break the category of bisexual behavior from same-sex behavior.
“Because that’s a controversy — is sexuality discrete or continuous? Do they have [a] different genetic background or do they have the same genetic background?” Zhang said.
The answer to that question revealed something unexpected: evidence of different genes associated with bisexual and exclusively same-sex behavior – and the genes associated with bisexual behavior were also associated with having more children, although not the genes associated with same-sex behavior are strictly related.
This represented a partial resolution of the paradox of bisexual behavior — while also reaffirming the paradox of strictly homosexual behavior.
Next, Song and Zhang looked to correlate other characteristics from the Biobank survey responses with bisexual attraction.
They found that a higher appetite for risk-taking was associated with genetic factors related to bisexual attraction and having more children – a link so strong that, when they controlled for it, the an apparent connection between his bisexual attraction and more children.
“People who carry bisexual genes have more children. And the reason they have more children is the so-called 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 said. “They will have more children there. That’s what the results suggest.”
However, whatever the link between the genetic factors of bisexual orientation, risk and having more children, Zhang and Song’s past reports suggest that its effect has now largely disappeared. contraceptive crop.
In a study published by the pair last year in the Proceedings of the National Academy of Sciences (PNAS), a key aspect of Zietsch’s findings was discovered – that people carrying genes associated with same-sex attraction had more children.
“But it’s very difficult for that to work in modern society, because more sexual partners doesn’t necessarily mean more children, since 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 children came from more sex, and with it, a greater spread of the theoretical genetic basis for same-sex attraction.
“But after the 1960s, this relationship disappeared,” Zhang said. The data suggested, he said, that if an ancestrally linked trait led to same-sex attraction and more children, that link did not survive universal access to contraception, in the UK on the least.
The widespread availability of the pill, Zhang and Song wrote in PNAS, “could put an end to the aforementioned mechanism of genetic maintenance [same-sex behavior].”
Zhang noted that there is now a countervailing force against the contraceptive effect: the historical ability of same-sex couples to have their own children.
But he argued that’s not a big factor: It’s true that homosexuals have, on average, 75 percent fewer children than heterosexuals, and bisexuals have about 30 percent fewer, he said. he.
“We predict that exclusive same-sex behavior will decrease in frequency over time in the future,” he said. “But because same-sex behavior is influenced more by environment than by genes, it is not known whether the number of people in this society will increase or decrease.”
He also admitted that these numbers depend on something very difficult to prove: that British survey respondents were telling the truth about — and were fully aware of — their own same-sex attractions, rather than their actions.
He noted that surveys are highly unreliable.
That reliability problem disappears if you accept that survey errors are random — that as many bisexual people will “misreport” as homosexual, or heterosexual as bisexual, he said.
But if the errors are not random — if, for example, same-sex people are more likely to self-identify as heterosexual — bisexual males could have the same number of children as strict heterosexuals, if which is not more.
If so, that would undermine the idea that there is a Darwinian paradox, suggesting that there is a simpler link between same-sex attraction and having more children.
Zhang also mentioned a more obvious issue with the study: that the data set is based in the United Kingdom, among mostly European participants.
“So we don’t know if our findings apply to other populations,” Zhang said.
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