The idea that sexuality is tied to biology is nothing new: Scientists have been looking for the "gay gene" for decades, and more recent research has demonstrated that boys with older brothers are more likely to grow up to be gay. Now, we may know why.
According to a new study published in the Proceedings of the National Academy of Sciences, the “fraternal birth order effect," as the younger-brothers-are-more-likely-to-be-gay thing is called, may be caused by a protein. Andy Coghlan from New Scientist explains:
Anthony Bogaert at Brock University, Canada, and his team think that some women who are pregnant with boys develop antibodies that target a protein made by the Y chromosome. Our immune systems make antibodies to recognise foreign molecules, which have the potential to be from dangerous bacteria. But pregnant women sometimes also produce antibodies against fetal molecules – for example, if their fetus has a different blood group. Bogaert’s team wondered if maternal antibodies might play a role in shaping sexual orientation.
The team collected blood from 142 women, and screened it for antibodies to a particular brain protein that is only made in males. They thought this would be a good candidate, because it plays an important role in how neurons communicate with each other, and because it is produced on the surface of brain cells, making it relatively easy for antibodies to find and detect it.
They found that the mothers of gay sons with older brothers had the highest levels of antibodies against this protein, followed by the mothers of gay sons with no older brothers. Women who had straight sons had less of these antibodies, while women with no sons had the least.
It's important to note that the sample size is small, correlation does not equal causation, and there are plenty of other factors that may cause an increase in homosexuality; for instance, exposure to show tunes in utero, being forced to play Little League against your will, and walking in on your parents during sex.* But if this study can be replicated, it could mean that specific molecules may be implicated for both homo and heterosexuality, as Dean Hamer, a researcher of the biological determinants of sexual orientation, told New Scientist. And this, Hamer says, "could pave the way to a detailed neurobiological and genetic understanding of this fascinating aspect of human development."
Regardless of the cause of homosexuality, in contemporary thinking, sexuality is often considered a sort of immutable human trait. This was the basis for the 2015 Supreme Court decision Obergefell v. Hodges, which legalized same sex marriage. The court wrote: "Far from seeking to devalue marriage, the petitioners seek it for themselves because of their respect—and need—for its privileges and responsibilities. And their immutable nature dictates that same-sex marriage is their only real path to this profound commitment." It's nature versus nurture, and according to the court, nature wins. But, when you look at twin studies, the data is less clear. From a 2016 review of the research, published in the Journal of Sex Research:
Across the most reliable twin registry studies available, the median concordance estimate for sexual orientation among identical twins is .25. In summary, despite the fact that identical twins share 100% of their genes, gay/gay twin pairs are less common than gay/straight twin pairs. The twin data clearly show a genetic contribution to sexuality (because even a twin concordance as low as .25 is significantly greater than would be expected by chance, given the low base rate of same-sex sexuality in the population), but not genetic determination (which would produce perfect concordance in identical twins).
In other words, nature—be it a gene or a protein—counts, but the research suggests that nurture does, too. Like everything in life, sexuality is just less cut-and-dried than we often think.
*Kidding. Don't @ me.