In my last post, I invited you to submit your questions to me about any aspect of human behaviour that you'd like to know more about but hadn't the time, energy, or know-how to track down the answers. Quite a few of you responded, on topics ranging from the best treatment for spider phobias to whether "evil" people are self-aware. There were many interesting questions for me to choose from, but because several questions dealt with one particular topic - the natural foundations of homosexuality - I've decided to focus here on that issue. I should say first, though, that this is an enormously complex topic with a vast literature behind it, so I'll just pick out what I think are some of the more interesting empirical facts. And by the way, think of this not so much as a "pro-gay" post but more an "anti-stupid" post.

I'm certainly not the only evolutionary-minded psychologist who happens to be gay, but some people find it surprising that a homosexual male would be sympathetic to a theoretical framework that seems to suggest he's a biological anomaly. One needn't a very deep understanding of Darwinian concepts to get the gist of the irony: at the heart of natural selection theory is genetic replication, and the best way to promulgate one's genes is to engage in reproductive sex, something that people of my ilk often consider to be rather, well, icky. Yet, as we'll see, genetic replication can occur by means other than direct reproduction. I've a slew of straight biological relatives who share my genetic material, and their reproductive success is my reproductive success.

But let's back up a bit. First, one reader ("ill") asked me to address whether LGBT people (this acronym always reminds me of a sandwich) are born this way or whether they make a conscious/unconscious decision to be so. This is a very important question because oftentimes it's embedded in moralistic debates, which in turn serve to inform policy. It's also a question that's especially prone to the naturalistic fallacy, a logico-deductive reasoning error that erroneously implies that what is natural is good. The argument goes something like this: If people choose to be gay, they're choosing to go against nature, therefore being gay is wrong. Never mind the fact that occasional homosexual activities occur in more animal species than not and are in fact perfectly compatible with nature, but by this logic, brushing our teeth and wearing deodorant are also morally wrong because they're both unnatural deeds. Yet, strangely enough, neither the religious right nor puerile homophobes seem to be as focused on these flagrant offences of the natural olfactory order.

But for those who still insist on subscribing to the naturalistic fallacy, what you're basically telling us is that, if you really wanted to, you could choose to get an erection by watching another man (well, at least a good-looking man) get naked. You just choose not to, is that it? On that note, here are some scientific tidbits about homophobia:

• A study by Henry Adams and his colleagues at the University of Georgia showed that, on a penile plethysmograph measure, aggressively homophobic men tend to get sexually aroused by explicit gay male pornography.

• According to Vanderbilt psychologist Bunmi Olatunji and his colleagues, homophobia is a misnomer, given that aversion to gays does not represent an actual phobia, but is rather driven primarily by disgust and irrational contagion beliefs.

• Similarly, Cornell researcher David Pizarro (who has his own blog column here at PT, incidentally) further revealed in a recent study that individuals who have a low-disgust sensitivity threshold (that is, they're easily grossed out) are more likely to hold both conservative attitudes and to be averse to gays.

• Based on survey studies, evolutionary psychologist Gordon Gallup has argued that homophobic reactions are rooted to people's folk theory that children "learn" to be gay by being exposed to gay adults. Thus, because parents have a vested genetic interest in the reproductive status of their biological offspring, homophobia is exacerbated whenever gay people have contact with children.

It's never been clear to me why, even if it were true that being gay came down to personal choice, it would be any less OK than if it were completely biologically determined. In fact, this isn't likely to put me on good terms with the ACLU, but there's some reason to believe that not all gay people are "born" gay. And that's simply this: there are many examples where one identical twin is homosexual and the other isn't. If it were as straightforward as a genetic cause, identical twins (who, if it hasn't registered yet, are genetically identical) would be perfectly concordant on this trait. So, although there is indeed a strong component of heritability for sexual orientation - identical twins do have a moderately significant rate of concordance for homosexuality, it's just not perfectly concordant - obviously it can't simply be boiled down to a "gay gene."

Does this mean it's a choice after all? Hardly. Just like any complex trait, homosexuality is almost certainly a matter of having a genetic predisposition to turn out gay, but requiring a specific developmental milieu to be expressed phenotypically. (Nancy Segal, are you out there somewhere in the PT blogosphere?)

Some of the most important environmental variables identified by researchers as being correlated with homosexuality are factors in the prenatal environment, which is a topic that another reader ("Leah") asked me to discuss. Remember that development doesn't just start on your birthday, but before you ever shimmied into this world through you mom's birth canal you had nine months of the prenatal environment to shape and influence your brain.

When it comes to homosexual orientation, several findings have indicated a reasonably strong prenatal effect. First, the more older biological brothers (but not stepbrothers) a boy has, the more likely it is that he will be gay. This fraternal birth order effect is explained by the maternal immune hypothesis: some mothers appear to become progressively immunised against male-specific antigens by each succeeding male foetus. This maternal immunisation, in turn, means that her anti-male antibodies increasingly interfere with the sexual differentiation of each succeeding male foetus's brain. (Recall that the "default" gender in prenatal development is female.) Interestingly, recent findings indicate that the fraternal birth order effect occurs only for right-handed males.

Prenatal hormonal effects also appear to leave specific "signatures" on physical development. One of the best bio-demographic makers of sexual orientation is the length of the index finger relative to the ring finger (otherwise known as the 2D:4D effect). How's this for a gaydar heuristic? Straight men and lesbians most often have shorter index fingers than ring fingers. This is proposed to be the product of high prenatal testosterone. But for gay men, just like straight women, this digit ratio difference is smaller, and sometimes even reversed. This reverse 2D:4D trend tends to be more pronounced the more effeminate the gay man, and interestingly also predicts preference for receptive anal intercourse (in other words, the longer a gay man's index finger and the shorter his ring finger, the more likely he is to be a "bottom").

Let me wrap up by mentioning a few of the very creative theories that scholars have come up with to explain homosexuality from an evolutionary perspective, which is something "Phill" wanted me to discuss. Remember, being gay seems to fly in the face of natural selection theory, and one might expect nature to have aggressively selected against any genetic substrate that lent itself to such a blatantly anti-reproductive trait. I can't possibly cover all of the vicissitudes of these evolutionary theories, nor address all of them, but I'll summarise the central points. By the way, virtually all theories in this area focus on male homosexuality; lesbianism has been almost completely ignored by evolutionary theorists.

As evolutionary psychologist David Buss has said, homosexuality is a "genuine evolutionary puzzle," so take your pick among these possible solutions (or come up with your own):

• E.O. Wilson's kin altruism theory states that homosexuality was a rare but functional alternative to traditional routes of increasing inclusive fitness because gay people in the ancestral past, who weren't burdened with their own kids, helped to raise, care for, and provide resources to their other genetic relatives, such as nieces and nephews. (This one doesn't quite gel, especially when you consider that a gay person's resources are usually funneled to their same-sex partners. Also, for most people, being gay doesn't exactly endear you to your relatives.)

• Evolutionary psychologist Frank Muscarella's alliance formation theory proposes that, in the ancestral past, homoerotic behaviours by young men with high status older men would have been an effective strategy for climbing up the social ladder. (Think Ancient Greece, or maybe Mark Foley?)

• John Maynard Smith is often credited with what is colloquially called the "sneaky f*cker theory," which argues that gay men in the ancestral past had unique access to the reproductive niche because females let their guards down around them and other males didn't view them as sexual competitors. (I rather like this one: remember, we're not infertile, we're just gay. Although in my case, it'd take a lot of gin to work.)