A new study has found that men who carry genetic factors linked to bisexual attraction tend to have more children.
The paper, published Wednesday in the journal Science Advances, found evidence of genetic variations that contribute to both bisexual attraction and increased risk appetite in men.
While these findings help resolve a paradox at the heart of evolutionary theory, they also raise difficult new questions about the genetic roots of same-sex attraction.
For generations, evolutionary biologists have viewed homosexual or same-sex attraction as a gap in evolutionary theory; Because it is an inherited trait, at least to some extent, it also causes those who have it to have fewer children.
“So if you put those two things together, it doesn’t make any sense,” lead author Jianzhi Zhang of the University of Michigan told The Hill.
“These genes lead to fewer children, which means they are selected by natural selection in the population. So they should gradually disappear from the population. So why are they still there?”
This is a question that goes far beyond the world of human sexuality. Homosexual attraction and behavior are common throughout the animal kingdom; from male gentoo penguins rearing eggs together to sex between herds of all-male bachelor gorillas and “seasonally bisexual” flying fox bats.
“No matter how you look at it, that’s it. “I can give you articles about insects, spiders, flies, fish, flamingos, geese, bison, deer, gibbons, bats,” ecology and evolution researcher Jackson Clive said in an interview with Imperial College London.
“There are lots of bats, all kinds of bats,” Clive added. “The list is endless.”
This is an exaggeration: In fact, the list is very long and includes more than 1,500 species, according to the Nature Ecology and Evolution study.
Theories aimed at finding a solution to the paradox are quite common and include arguments that there is no paradox. In the study Nature Ecology and Evolution, for example, the authors argue that scientists’ assumption that opposite-sex attraction is normal and ancestral has “not been rigorously examined.”
Instead, this team hypothesized “an ancestral state of indiscriminate sexual behavior towards all genders.”
In an article in Scientific American, 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 idea of paradox recognizes.
“Other traits required to recognize a compatible mate (e.g., differences in size, shape, color, or scent) are unlikely to have evolved at exactly the same time as sexual behaviors,” they wrote.
“In fact, indiscriminate mating may be more beneficial than costly.”
Another study, also published in 2023 in the journal Nature Ecology and Evolution, found that male macaque monkeys mounting each other strengthened their relationships in a way that “mediated coalitional partnerships linked to increased reproductive success.”
Or, as a separate Scientific American article about three “lesbian” capuchin monkeys at a Los Angeles sanctuary noted in September, “it is clear that being a little gay almost every day helps primates have their own way in pleasure and life.”
But others suggest that the solution to the paradox lies in a concept called “pleiotropy,” in which the collection of genes that lead to one trait also leads to another trait.
Many scientists have suggested that the fact that same-sex attraction can be inherited makes it an “antagonistic pleiotropy,” that is, a shared gene giving rise to different behaviors that work against each other to increase and decrease the number of offspring.
According to this hypothesis, a gene that leads to bisexual attraction and therefore having fewer children may still be preserved if it is also linked to another trait that leads to having more children.
For example, scientists have suggested that genes for same-sex attraction may lead to more shared parenting (or a cast of same-sex uncles and aunts who do not have children), or that male homosexuality may be the spread of genetic traits that make men heterosexual. females are more fertile.
But Zhang said these hypotheses share a common problem: “Most of them lack empirical evidence.”
But in 2021, researchers published a study in the journal Nature Human Behavior that provides empirical evidence for the idea that same-sex attraction may be linked to other traits that increase the number of children a person has.
By examining the UK Biobank, a massive database of 450,000 genetic samples linked to detailed behavioral studies, researchers found a number of genetic variants associated with same-sex behaviour.
Then, by comparing these variants to the reported number of sexual partners of both sexes of the person carrying them, they found that “genes associated with having a same-sex partner were also associated among people who had never had a same-sex partner.” sex partners — along with having more opposite-sex partners, as co-author Brendan Zietsch explains in an article in The Conversation.
Although the mechanism of how this works is unclear, Zietsch’s team suggested that these factors could work together to “make someone ‘more attractive’ in a broad sense.”
In the study published Wednesday, Zhang and lead author Siliang Song began mining UK Biobank data for a more definitive mechanism.
Their first discovery came when they tried to see if they could separate the category of bisexual behavior from full-blown homosexual behavior.
“Because this is a debate; Is sexuality discrete or continuous? Do they exist? [a] Do they have different genetic bases, or do they have the same genetic base?” said Zhang.
Answering this question revealed something unexpected: evidence of different genes associated with bisexual and same-sex-only behavior; Genes linked to bisexual behavior were also associated with having more children, while those linked to strictly same-sex behavior were not.
While this indicates a partial resolution of the paradox surrounding bisexual behavior, it also seemingly reaffirms the paradox when it comes to strictly homosexual behavior.
Song and Zhang then looked for other traits that were associated with bisexual attraction from the Biobank survey responses.
They found that increased risk appetite was associated with genetic factors linked to both bisexual attraction and having more children; this connection was so strong that when they controlled for it, the apparent link between bisexual attraction and having more children disappeared.
“People who carry bisexual genes have more children. The reason they have more children is their so-called bisexual genes. [mean that they] “We are willing to take more risks,” Zhang said.
Zhang added that for some people, “more risk” will mean more sexual partners. “Then they will have more children. The results show this.”
However, whatever link exists between bisexual orientation, risk, and genetic factors associated with having more children, Zhang and Song’s past reports suggest that its influence is now largely obsolete due to birth control.
In a study published last year in the Proceedings of the National Academy of Sciences (PNAS), the duo addressed a key element of Zietsch’s findings: people who carried genes linked to same-sex attraction had more children.
“But it’s very difficult for this to work in modern society, because having more sexual partners doesn’t necessarily mean more children, given that we have birth control,” Zhang said.
Examining the biobank, Zhang and Song found that before the 1960s, when oral contraceptives, or “the pill,” became commonplace in Britain, more sex did indeed lead to more children, and with it the theoretical genetic basis of birth spread further. homosexual attraction.
“But after the 1960s, this relationship disappeared,” Zhang said. The data suggested that if a linked ancestral trait led to both same-sex attraction and having more children, this link did not survive universal access to birth control, at least in the UK.
The widespread availability of the pill “may eliminate the aforementioned mechanism for genetic conservation,” Zhang and Song wrote in PNAS. [same-sex behavior].”
Zhang noted that there is now a countervailing force to the influence of birth control: the historically new ability for same-sex couples to have children of their own.
But he argued that wasn’t a big factor: Those who were purely homosexual had, on average, 75 percent fewer children than those who were entirely heterosexual, he said, while bisexuals had about 30 percent fewer children.
“We predict that the frequency of homosexual-specific behavior will decrease over time in the future,” he said. “But since homosexual behavior is influenced by environment rather than genes, it is unknown whether the proportion of people in this population will increase.”
He also acknowledged that these figures were based on something very difficult to prove: that British survey respondents were telling the truth about their same-sex attraction and were fully aware of it, contrary to their actions.
He noted that polls are notoriously unreliable.
This reliability issue disappears if you assume survey errors are random, he said, meaning many bisexual people will “misreport” as gay or heterosexual people as bisexual.
But if the errors are not random (for example, if same-sex orientation people are more likely to mistakenly identify themselves as heterosexual), then bisexual men may have the same number of children as strictly heterosexuals, if not more.
If so, this would refute the idea of a Darwinian paradox and suggest a simpler connection between same-sex attraction and having more children.
Zhang also noted a more obvious problem with the study: The data set was based in the United Kingdom, with mostly European participants.
“So we don’t know if our results apply to other populations,” Zhang said.
Head to The Hill for the latest news, weather, sports and streaming video.