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For an evolutionary biologist, homosexuality is something of a puzzle. It’s a common trait, found in up to 10% of the population. It appears to be run in families, suggesting that it is hereditary, at least in part. And yet it defies the very reason why traits are passed on from generation to generation. How could something that hinders childbearing be passed down so frequently from parents to children?

Researchers at the National Institute for Mathematical and Biological Synthesis (NIMBioS) think they may have an answer. It’s not in written in our DNA sequence itself, they suggest, which explains why scientists have failed so far to find “gay genes,” despite intensive investigations. Instead, it’s written in how our genes are expressed: that is, in certain modifications to how and when DNA is activated. These changes can have environmental roots, so are not normally permanent enough to be passed from parent to child. But occasionally, they are.

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“It’s not genetics. It’s not DNA. It’s not pieces of DNA. It’s epigenetics,” says Sergey Gavrilets, a NIMBioS researcher and an author on the paper that outlines the new theory of homosexuality, published in The Quarterly Review of Biology. “The hypothesis we put forward is based on epigenetic marks,” he says.

To be specific, the new theory suggests that homosexuality is caused by epigenetic marks, or “epi-marks,” related to sensitivity to hormones in the womb. These are compounds that sit on DNA and regulate how active, or inactive certain genes are, and also control when during development these genes are most prolific. Gavrilets and his colleagues believe that gene expression may regulate how a fetus responds to testosterone, the all-important male sex hormone. They further argue that epi-marks may help to buffer a female fetus from high levels of testosterone by suppressing receptors that respond to testosterone, for example, (thus ensuring normal fetal development even in the presence of a lot of testosterone) or to buffer a male fetus from low levels of testosterone by upregulating receptors that bind to the hormone (ensuring normal fetal development even in the absence of high levels of testosterone). Normally, these epi-marks are erased after they are activated, but if those marks are passed down to the next generation, the same epi-marks that protected a man in utero may cause oversensitivity to testosterone among his daughters, and the epi-marks that protected a woman in utero may lead to undersensitivity to testosterone among her sons.

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Gavrilets says that some scientists have already expressed “strong interest” in new experiments that will test the hypothesis, attempting to estimate how often such epi-marks may arise among men and women, and how often they are saved from one generation to the next. The work might also explain the extent to which epi-marks can influence sexual behavior. While experimental evidence is lacking for now, Gavrilets says he is reasonably confident that the theory is sound.

“It’s compatible with the [existing] data. Plus it’s supported by mathematical modeling,” he says.

The new theory is important because it synthesizes well-tested and well-developed evolutionary principles with cutting-edge research in molecular biology and biological computation. Epigenetics is not a new concept exactly, but the field has exploded within the past decade. Where once it seemed that genes and environment were distinct, or that nature and nurture were distinct, now it seems clear that environment itself may change the ways in which our genes function – even though the genes themselves are essentially fixed over time, barring occasional mutations, and conserved across generations.

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For now, the work is still controversial. Gavrilets says he and his colleagues have been criticized both by conservatives, who weren’t happy with people searching for a biological basis to homosexuality, as well as by some in the gay and lesbian community, who feel they shouldn’t require a scientific investigation to justify who they are.

“It’s been a controversial topic,” Gavrilets says. “There is a lot of misconception in the world about the topic of homosexuality.” Still, he says, “One of the roles of science is to clear up misconceptions, to explain different phenomena or different patterns.”