by Men who have genetic factors linked to bisexual attraction tend to have more children, according to a new study.
The paper published Wednesday in Science Advances found evidence of genetic variations that contribute to both bisexual attraction in men and a greater appetite for risk.
Those findings help resolve a central paradox of evolutionary theory, while raising difficult new questions about the genetic roots of same-sex attraction.
For generations, evolutionary biologists have viewed homosexual or same-sex attraction as a hole in evolutionary theory, since 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 produce fewer children, which means they are being selected out of the population through natural selection. So, little by little, they should disappear from the population. So why do they still exist?
That is a question that goes far beyond the world of human sexuality. Same-sex attraction and behavior is widespread throughout the animal kingdom, from male gentoo penguins raising eggs together, sex between pods of all-male bachelor gorillas, and “seasonally bisexual” flying fox bats.
“It’s everywhere you look. “I can give you articles about beetles, spiders, flies, fish, flamingos, geese, bison, deer, gibbons and bats,” said ecology and evolution researcher Jackson Clive in an interview with Imperial College London.
“There are lots of bats, bats do all sorts of things,” Clive added. “The list is endless.”
That’s an exaggeration: in fact, the list is simply too long, including more than 1,500 species, according to a study in Nature Ecology and Evolution.
Theories for a solution to the paradox are widespread and include arguments that no paradox exists. In the study Nature Ecology and Evolution, for example, the authors argue that scientists’ assumption that attraction to the opposite sex is normal and ancestral “has not been rigorously examined.”
Instead, that team hypothesized “an ancestral condition of indiscriminate sexual behaviors directed at all sexes.”
In an essay in Scientific American, the Nature authors argued that the foundations of animal sex may have been laid long before the relatively clear physical differences between males and females that the paradox idea takes for granted.
“It is unlikely that the other traits necessary to recognize a compatible mate (differences in size, shape, color, or smell, for example) evolved at exactly the same time as sexual behaviors,” they wrote.
“In fact, indiscriminate mating may be more beneficial than costly.”
Another 2023 study, also in Nature Ecology and Evolution, found that male macaque monkeys that mounted each other appeared to strengthen their relationship in a way that “mediated coalition associations that have been linked to better reproductive success.”
Or, as a separate Scientific American article about three “lesbian” capuchin monkeys at 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 resolution of the paradox lies in a concept called “pleiotropy,” in which the set of genes that leads to one trait also leads to another.
Many scientists have postulated that the fact that same-sex attraction can be inherited makes it an “antagonistic pleiotropy,” a shared gene that leads to different behaviors that act on 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 linked to another trait that led to having more children.
For example, scientists have postulated that genes for same-sex attraction could lead to greater shared parenting (or a caste of childless same-sex uncles and aunts) or that male homosexuality could be an extension of the genetic traits that caused heterosexuals to become heterosexual parents. more fertile females.
But these hypotheses share a common problem, Zhang said: “Most of them have no empirical evidence.”
In 2021, however, researchers published a study in Nature Human Behavior that offered empirical evidence for the idea that same-sex attraction could be related to other traits that increase the number of children a person has.
Examining the UK Biobank, a massive database of 450,000 genetic samples associated with detailed behavioral surveys, the researchers found a number of genetic variants that correlated with same-sex behavior.
Then, by comparing those variants to the reported number of sexual partners of both sexes that the person carrying them had, they found that the genes “associated with having ever had a same-sex partner were also associated (among people who had never had the same couple). sexual partner: having had more partners of the opposite sex,” as co-author Brendan Zietsch explained in an essay in The Conversation.
While the mechanism by which this worked was unclear, Zietsch’s team speculated that these factors could work together to also “make someone ‘more attractive’ in general terms.”
In the study published Wednesday, Zhang and lead author Siliang Song looked to UK Biobank data for a more precise mechanism.
Their first discovery came when they tried to see if they could separate the category of bisexual behavior from strictly same-sex behavior.
“Because there is a controversy: is sexuality discrete or continuous? 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; Genes linked to bisexual behavior were also linked 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 seemingly reaffirming the paradox as it relates to strictly homosexual behavior.
Song and Zhang then looked for other traits in the Biobank survey responses that correlated with bisexual attraction.
They found that a greater appetite for risk-taking was associated with genetic factors linked 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 children disappeared.
“People who carry bisexual genes have more children. And the reason they have more children is because the so-called bisexual genes [mean that they] 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 there is between genetic factors associated with bisexual orientation, risk and having more children, Zhang and Song’s previous reports suggest that their effect is now largely obsolete due to contraception.
In a study the couple published last year in the Proceedings of the National Academy of Sciences (PNAS), they delved into a key element of Zietsch’s findings: that people who carried genes related to same-sex attraction had more children. .
“But it’s very difficult for that to work in modern society, because having more sexual partners doesn’t necessarily mean more children, given that we have contraceptives,” Zhang said.
When they examined the Biobank, Zhang and Song discovered that before the 1960s, when oral contraceptives or “the pill” became widespread in England, more sex actually led to more children and, with it, a greater spread of the base Theoretical genetics of same-sex attraction.
“But after the 1960s, this relationship disappeared,” Zhang said. The data, she said, suggested that if a linked ancestral trait had led to both same-sex attraction and having more children, that link had not survived universal access to contraception, at least in the UK.
Widespread availability of the pill, Zhang and Song wrote in PNAS, “could abolish the aforementioned mechanism for genetic maintenance of [same-sex behavior].”
Zhang noted that there is now a force counteracting the effect of contraception: the historically novel ability of same-sex couples to have their own children.
But he argued that’s not an important factor: Strictly gay people have 75 percent fewer children on average than strictly straight people, while bisexual people have about 30 percent fewer, he said.
“We predict that, in the case of same-sex exclusive behavior, its frequency will decrease over time in the future,” he said. “But because same-sex behavior is more influenced by environment than genes, it is unknown whether the proportion of people in this society will increase or decrease.”
He also acknowledged that these figures are based on something very difficult to prove: that British respondents were telling the truth about (and are fully aware of) their own same-sex attractions, as opposed to their actions.
Polls, he noted, are notoriously unreliable.
That reliability problem disappears if you assume that the survey errors are random: that the largest number of bisexual people will “misreport” as gay, or straight people as bisexual, he said.
But if the errors are not random (if, for example, people with a certain same-sex orientation are more likely to misidentify as heterosexual), then bisexual men could have the same number of children as strict heterosexuals, if not further.
If so, that would undermine the idea of a Darwinian paradox, suggesting a more direct link between same-sex attraction and having more children.
Zhang also pointed out a more obvious problem with the study: that the data set is based in the United Kingdom, among largely European participants.
“So we don’t know if our results apply to other populations,” Zhang said.
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