Was thinking about this today:
Do scientists have a good idea of what causes homosexuality or no?
To lay-people there doesn't seem to be consensus, but I assume we've got this one nailed down.
I think that's a poor assumption.
We know in the very broadest terms what causes sexual attraction - 'Hormones'
We even know to some extent which hormones - Oxytocin is clearly very important, for example.
What causes people to be sexually attracted to certain characteristics, or to certain individuals? I don't believe anyone knows. It's very difficult to even develop sensible protocols to study such a thing. Asking people what they find attractive results in a long list of apparently unrelated stuff, some of which is contradicted by their actions; the act of trying to observe people's actions is liable to introduce confounding factors, and double blind studies of people's sexual behaviour - ie spying on people - are generally considered unethical.
It's not even clear to me that 'homosexuality' is a useful category. If John has ten sexual partners, and all of them are men, we call him 'homosexual'; but how useful is that as a label? What if all of those ten partners are also slim, blond, muscular, introverts with an interest in fine art? Why is their gender so important that it defines who John is, but their other characteristics are considered trivial?
People are not attracted to gender
per se; I have never met a person who finds every single member of a given gender sexually attractive. People are quite picky about their sexual partners - some more so than others - and gender is just one of the myriad characteristics people use when deciding whether or not a given individual is attractive to them. If an obese person only dates slim women, is he or she heteroadiposal? If he or she only dates other obese people, are they a homoadiposal couple? Is this a valuable way to categorise people?
Obviously if long-term pair bonding occurs in a species (as it does to some extent with humans), there is some evolutionary pressure towards heterosexual partnerships; There is also evolutionary pressure towards abandonment of infertile partnerships - but for some reason, we decide that heterosexuality is 'normal' and that ditching your partner just because you haven't had kids after being together for a while is 'not normal'. So this is not about appealing to nature (which is fallacious anyway - 'natural' neither implies 'normal' nor 'right') - the elevation of gender specific sexuality as an important and even defining characteristic is a social convention, not a natural law.
It seems that the specifics of what people find attractive in a partner are mostly genetic, with the genotype influencing the endocrine system phenotype such that most people are less attracted by their own gender than by the other; But as with most genetics, the details are obviously hugely complex. Gregor Mendel's neat idea of dominant and recessive gene pairs linked directly to one phenotypic characteristic are very rare indeed. Most traits are mediated by complex interactions of large numbers of genes, with epigenetic and even endocrine feedback loops, and this is particularly true of gender - the huge morphological and physiological differences between men and women all stem from a tiny number of genes on the Y chromosome, which if present, turn on and off all the vast numbers of other genes needed to make a male, rather than an female. This process is highly complex, and often incomplete; Male <--> Female is a continuum, with concentrations of individuals at each end, but a sizeable minority somewhere in the middle.
In short - this is a hugely complex and poorly understood system; and we are not even able to say for sure whether the term 'homosexuality' refers to something objectively real. If John has had ten sexual partners, all of them male, does this information allow us to be 100% certain that his next choice of sexual partner will not be female? I don't think it does - it seems likely, but it is far from a certainty. If his eleventh partner is female, does that tell us anything about the likely gender of his twelfth? Does that suddenly change his status from homosexual to heterosexual? Or to bisexual?
Never mind what causes homosexuality - I don't think we have even nailed down a useful definition of what homosexuality IS.
