Proof we are not blank slates: Dogs can't speak English. No matter how much exposure to English you give a dog, it will never be able to converse with fluent syntax. (It may be able to understand a few English words, which an earthworm could not do; but it won't be able to have a conversation with you the way most humans could.)

Proof no trait is entirely monogenic: Even a Mendelian trait will only express itself in a particular organism; the genes for "yellow peas", if injected into a human being, will not turn a human being yellow, and the genes for cystic fibrosis, if injected into a pea, will not give that pea cystic fibrosis (what would that even mean?). Furthermore, genes can be activated and deactivated by environmental triggers.

Proof no trait is entirely environmental: Even a behavior that must be trained and learned with considerable effort—mathematics, tennis, as above, English—requires a substrate of biological and neurological capacities in place in order for learning to occur.

It must be both. It must always be both. We must learn to accept this and deal with its consequences.

To take a particular example from human life which has caused much pain and controversy: Is homosexuality, in either attraction or behavior, genetic or environmental? Both. It must be both. The question is which genes and which environmental triggers do what and how; but clearly there are genes and environmental triggers involved.

Could there be a "gay gene"? To some extent, yes. Such a gene would have to be a gene for some sort of protein that, when synthesized in the human brain, changes the balance of hormones in order to trigger changes in the brain that alter one's perception of gender or sexual attraction. But this itself is problematic, since all human decisions are made in a complex social environment. A person with this same gene in a completely different environment—say, isolated from other humans, or in a highly polyamorous society—might not end up behaving in a way that we would recognize as "homosexual". Furthermore, given a different configuration of other genes, this gene—and the protein it produces—might have a completely different effect; a gene that in some people produces homosexual attraction could in other people slow aging or increase aggression or whatever else. These genes could also be expressed or unexpressed, or activated and deactivated at different points in development based on environmental triggers.

But nor can we say that "society makes us gay", since there could be particular genotypes such that, if you have these genes, virtually no environmental circumstance would ever cause you to experience homosexual attraction. There could be other genotypes that are much easier to sway environmentally, and still other genotypes for which homosexual behavior is the inevitable and incorrigible result. The evidence for "ex-gay" people is at present rather scant, but even if it does turn out that there are many people who have been able to change from homosexual to heterosexual behavior or even attraction through therapy, that only places limits on genetic hypotheses of sexual orientation (that in at least some cases, any genes that affect sexual orientation can be environmentally modulated) it would not undermine them entirely (for there still would be genes necessary for these environmental pressures to have their effect).

Now, it is a valid question how much of the variance in human sexual orientation is genetic, and how much environmental. My own suspicion is that the majority is genetic, since many human beings display homosexual and bisexual attraction and behavior even in environments that strongly pressure them toward heterosexuality, and because there is so little evidence in favor of people changing from homosexual to heterosexual behavior. Still, there are likely to be many environmental factors as well, as the presence of situationally homosexual behavior in humans and other animals attests. There are also many social issues involved in how we recognize sex and gender, which will no doubt be environmentally modulated.

Indeed, we may occasionally experience sexual attractions in tension with our normal orientation, due to the fact that we have no direct perception of gender itself—only perception of features associated with gender, such as appearance and behavior. For instance, many heterosexual males are frightened by the fact that they have experienced attraction to highly feminine males, but this is hardly surprising: If you are attracted to certain traits associated with femininity, then a male displaying those traits will attract you. Effectively, you have fallen prey to a perceptual illusion. Somewhere in your brain you have a female detection system, which uses certain perceptual features to detect attractive females; this male has many of those features, and so your detection system has registered a false positive. It could also be that there are certain features which are simply sexy per se, that will attract people regardless of whether they are possessed by males or females.

What significance does all of this have for the politics of sexual orientation? Not much, really. Even if it were demonstrable that 100% of the variance in sexual orientation is attributable to genetics, it could still be the case that homosexuality is morally harmful and ought to be resisted. (Imagine for instance if we found that 100% of the variance in genocide were explainable genetically; we would then consider genocide a genetic disorder—we would not conclude that genocide is morally acceptable.) Conversely, even if it were demonstrable that 100% of the variance in sexual orientation could be explained environmentally, this would not give us a reason to consider homosexuality immoral or worthy of change. (Nearly 100% of the variance in French fluency is explainable environmentally; does this make speaking French, or failing to speak French, immoral?) Of course, both of these extremes are ridiculous; clearly some of the variance will be genetic, and some environmental.

But in any case, the question is not "what causes it?" but rather, "Is it immoral?" And on this front I can see no reason to deny that homosexual behavior can be part of a fulfilling and morally just human life, in much the same way that heterosexuality, bisexuality, and asexuality can. Surely some behaviors are both homosexual and immoral, but this is inevitably for reasons that have little to do with sexual orientation (rape is a physical and emotional harm; adultery is a breach of contract).

True, there are some ancient books that say homosexuality is immoral—along with cattle breeding, beard shaving, wearing of heterogeneous fiber, eating shellfish (for once I agree, but for completely different reasons), and failing to kill your children when the voices say so. (Muslims around the world still celebrate a holiday annually called "Eid al-Adha" in celebration of the fact that Abraham did not shirk this latter duty.) These ancient books are a source of a wide variety of moral notions, from the eminently sound ("And as ye would that men should do to you, do ye also to them likewise." Luke 6) on through the mildly dubious ("an eye for an eye, a tooth for a tooth, a hand for a hand, a foot for a foot" Exodus 21) to the absurdly evil ("go and smite Amalek, and utterly destroy all that they have, and spare them not; but slay both man and woman, infant and suckling, ox and sheep, camel and ass." 1 Samuel 15). Literally everything from pacifism to genocide is advocated within the pages of books like the Bible and the Qur'an. If we are seeking a coherent moral system, we must seek elsewhere.

Of course the choice of what "elsewhere" to use is not obvious, and there has been much debate about it, between realists and expressivists, Kantians and utilitarians, liberals and conservatives. In future work I will explicate and attempt to prove my own conception of morality, but as yet I can at least say this: The whole of the mainstream philosophical community agrees that homosexuality is no more immoral than reading a book or riding a roller coaster. All these actions have null Darwinian fitness and take energy and time, but bring pleasure, at least to some people—and as such we ought to have the freedom to practice them.