-A curious adult from the U.S. July 2, 2013 Most of the research done so far suggests that in the vast majority of cases, being gay is not a choice for people. Instead, it appears to be a fundamental part of who they are. What this means is that most people cannot “unlearn” their sexual orientation. So in most cases, being gay is not a learned behavior. Of course just because we know it isn’t usually a learned behavior, that doesn’t mean that we have a good explanation for what is going on biologically. We don’t. What we do know is that there isn’t one single gene that explains homosexuality. Something as complicated as sexual orientation is going to involve lots of genes. And not only that, but it will involve the environment too. Now by the environment I don’t just mean an overprotective mom or a domineering dad. Environment is a catchall for everything that isn’t a gene. For instance, what the fetus was exposed to while in the mother’s womb can affect its development and influence behavior later on in life. So even though you might expect that the environment only causes temporary changes, that’s not always the case. The environment can cause brains to be wired in a certain way as it develops. This wiring can’t be changed easily. Right now the best guess is that people inherit a set of genes that makes them more or less likely to respond to something in the environment in a way that causes their brains to be wired so that they are attracted to people of the same sex. Whew! That was a mouthful! Given all this it sounds like it’d be a nightmare to figure out what is going on biologically. But scientists are starting to make some progress. The most convincing data about the genetic basis of homosexuality comes from looking at twins. Family studies are also giving us a few clues about certain environmental triggers.

Studying Twins to Learn Genetics Some of the best evidence that homosexuality has a strong genetic part comes from twin studies. These studies are useful because they let scientists tease apart how much of a trait is influenced by genetics and how much is influenced by the environment. They do this by comparing identical twins, who share the same DNA sequence, and fraternal twins, who are only about 50% related (as much as siblings). If a trait were completely genetic, then we’d expect identical twins to always be the same with regard to that trait. This is true for simpler, genetic traits like eye color. If one identical twin has blue eyes, then the other one will have blue most of the time too. But if a trait is mostly due to environment, then two identical twins would be just as likely to share the trait as two fraternal twins. Having the same DNA won’t matter because it doesn’t play a big role in that trait. Tongue rolling is like this. If your twin can roll his or her tongue, it doesn’t really affect your chances. This seems to be pretty independent of genes. So, if a certain trait happens more often between both twins in an identical twin pair compared to both twins in a fraternal pair, then genetics probably plays some role in that trait. This is because both fraternal and identical twins are born and raised in similar environments, so any differences seen between them are most likely due to their DNA. Twin studies of homosexuality have shown that identical twins are about twice as likely to both be gay compared to fraternal twins. This means that being gay is partly genetic and not simply something that a person learns or chooses to be. There is one important thing to note, though. If the DNA sequence is the only thing determining whether someone is gay or not, we would expect that if one identical twin were gay, then the other would be too 100% of the time. But this is not what scientists have found – the rate is actually closer to 50%. So while we know that genetics is involved, it doesn’t tell us the whole story. This is where environment comes in.