Homosexuality was studied , in the US at least , by psychologists for 30 or 40 years . This was mostly done to see if it was an actual mental illness which needed to be included in the Diagnostic and Statistics Manual of Mental Disorders , commonly known as the DSM , which psychiatrists and psychologists use to classify and diagnose mental disorders and illnesses . The studies showed that , no , homosexuality is not a mental disorder of some sort and so it was removed from the DSM . Homosexuality has also been studied on a genetic level , and I believe those studies are still being done . If I recall correctly , there does appear to be a genetic component but nothing definitive has been found . You can ' t point to a specific gene or mutation and say , Aha ! This person must be gay because of this gene . It ' s also studied in biology , though mostly through noting that it happens in numerous species . There was even a book published scientifically documented examples of homosexuality in more than 300 species other than humans . I believe it was Biological Exuberance Animal Homosexuality and Natural Diversity by Bruce Bagemihl , but it may have been another book . Then , of course , sociologists have almost certainly studied it . I ' m not aware of any specific studies , though , but it would surprise me if there weren ' t many . Having said all of that , there ' s still no consensus on why it happens other than it appears to be a combination of things . Genetics is one , as I mentioned . There also appears to be a link to prenatal conditions in the womb and hormone levels of the mother . Even birth order has been indicated if you ' re the youngest of numerous brothers you ' re statistically more likely to be gay , if I recall correctly . However , like I said , not much is really known about the cause .