Very off topic, but I don’t think they’ve really determined what causes homosexuality (that is, the predilection for a sexual attraction to one’s own gender, not homosexual activity) yet, though they’re coming up with some excellent theories - and there are a lot of not-so-excellent ones as well. I’m going to paraphrase a summary of the reasearch that I’ve found.
I can list a few theories that I have heard or seen written up at one time or another. In very approximate order of scientific respectability, as best I can judge it, the theories are:
(1) Satan. Homosexuality may be a manifestation of Satan’s work. While the least scientific of current theories, this one is probably the most widely believed, taking the world at large. Most devout Muslims, for example, believe it, and so do many Christians.
(2) Social Construction. There is no such thing as homosexuality. There are only heterosexual and homosexual acts, which different cultures regard differently. The notion of “homosexuality” as a personality attribute is a 19th-century invention.
(3) Brain damage. Some insult to the tissues of the brain, perhaps at birth or in infancy, causes homosexuality.
(4) Choice. People choose to prefer their own sex over the other.
(5) Family influences in childhood. The Freudian belief is that having a weak father and/or dominant mother can form the child’s personality in the direction of homosexuality.
(6) Social stress. Rats kept in overpopulated environments, even when sufficient food and access to females are available, will become aggressively homosexual after the stress in the environment rises above a certain level.
(7) Imprinting. The individual’s early sexual history can “imprint” certain tendencies on animals and humans. Many homosexuals report having been same-sexually molested in childhood or youth.
(8) Socialization theories. The high levels of homosexual bonding in some ancient and primitive societies suggests that the common mores of a culture have some power to socialize large numbers of people into homosexuality.
(9) Genetics, direct. Homosexuality is the expression of some gene, or some combination of genes.
(10) Womb environment ? too much of a good thing. The presence of certain hormone imbalances during critical periods of gestation can have the effect of hyper-masculinizing the brain of a male infant. Paradoxically ? there are plausible biological arguments ? this might lead to the infant becoming homosexual.
(11) Infection. Homosexuality may be caused by an infectious agent ? a germ or a virus. This is the Cochran/Ewald theory, which made a cover story for the February 1999 Atlantic Monthly.
(12) Genetics, indirect. Homosexuality may be an undesirable (from the evolutionary point of view) side effect of some genetic defense against a disease ? analogous to the sickle-cell anemia mutation, a by-product of genetic defenses against malaria, negative to the organism but nothing like as negative, net-net, as susceptibility to malaria.
(13) Womb environment ? too much of the wrong thing. Here the effect of the rogue hormones is to feminize the brain of a male infant. (I assume that there are theories corresponding to 10 and 13 for female infants, though I have never seen them documented - which actually calls into question whether differing mechanisms might cause male and female homosexuality) This is the one that made a splash the other day: BBC NEWS | Health | Womb environment 'makes men gay'
Note that theories number 9, 10, 12, 13, and conditionally (depending on the age at injury or infection) 3 and 11, could all be taken as saying that homosexuality is “inborn,” while only two of these six theories have anything to do with genetics. The confusion between “genetic” and “inborn” is epidemic among the general public, however, to the despair of those attempting to have a more nuanced discussion of the topic.
In the current state of our understanding, I don’t believe that anyone can say for sure what’s true – but, noting the link above, number 13 seems to be the front runner. I think that numbers 12 and 13 currently hold the strongest positions, with much, though I think declining, interest and research in 9 and 10, modest but growing interest in 11, and some lingering residual attachment to 6, 7, and 8.
The other theories are not taken seriously by anyone doing genuine scientific research. If anyone has information to the contrary written up in a respectable peer-reviewed journal of the human sciences, it would be new to me and something I might like to read (at least the summary).