It's also more likely for two identical twins, who share all of their genes, to both be gay than it is for two fraternal twins, who share just half of their genes, to both be homosexual. That study, involving more than 400 pairs of gay brothers, followed the 1993 report by geneticist Dean Hamer suggesting the existence of a 'gay gene.' Other research has found that being gay or lesbian tends to run in families. For instance, a 2014 study in the journal Psychological Medicine showed that a gene on the X chromosome (one of the sex chromosomes) called Xq28 and a gene on chromosome 8 seem to be found in higher prevalence in men who are gay.
But some genes may make being gay likelier. No studies have found specific 'gay genes' that reliably make someone gay. While the evolutionary purpose of this behavior is not clear, the fact that animals routinely exhibit same-sex behavior belies the notion that gay sex is a modern human innovation. And nearly 130 bird species have been observed engaging in sexual activities with same-sex partners. Lions, chimpanzees, bison and dolphins have also been spotted in same-sex pairings. For instance, female Japanese macaques may sometimes participate in energetic sexual stimulation. Humans aren't the only species that has same-sex pairings.
