Gayvolution

A recent study (hat tip Moby Kip) suggests that the brains of gay men have certain similarities to straight women, and the brains of gay women have similarities to straight men.

The brain features in question have their roots in early brain development, suggesting that environmental and social conditions played little roll in their formation. This is one more piece of evidence that homosexuality has causes in physiology and not culture.

That then raises the question of whether homosexuality has a genetic cause, which leads quickly to the question of how such a thing could be possible. After all, evolution favors traits that increase the likelihood of reproduction, which would seem to rule out homosexuality.

More to the point, evolution tends to eliminate traits which impair reproduction. Even if homosexuality is a lifestyle choice rather than a built-in preference, evolution should have pushed the development of the human brain to take away the choice. A brain that is resistant to homosexual urges would be more likely to reproduce. It should be an evolutionary advantage. So even if there isn’t a gay gene, there should be an anti-gay gene, and everyone should have it.

So, why are there still gay people?

Nobody knows for sure, but the most common speculation is that homosexuality is due to a recessive gene. In humans (and most animals) the genes for every genetic trait come in pairs. When a male and a female mate, each parent passes one of each pair of genes on to the child, which therefore has one randomly-chosen gene from each parent.

Typically, some of the genes are recessive, meaning that if the other half of the pair is different, they are never expressed—they have no effect on the individual carrying them. The classic example is blue eyes in humans: Blue eyes are recessive, so two brown-eyed people carrying recessive blue-eyed genes have a one-in-four chance of both passing on the blue-eyed gene and having a blue-eyed child. But if either one passes on the brown-eyed gene, the child will have brown eyes. On the other hand, two blue-eyed parents both have two blue-eyed genes, so their child can only have blue eyes.

(Caution: If you’re a blue-eyed man with a blue-eyed wife, and you’re now looking at your non-blue-eyed child with suspicion, don’t go making accusations just yet. There are other factors affecting eye color besides the blue/brown gene.)

If there is a recessive gay gene, a straight person could carry it without being gay. He or she would have a 50/50 chance of passing the gene on to a child, where it would also be recessive. The only way the recessive gene would be able to express itself and turn the person gay is if he or she received the same recessive gay gene from both parents.

A number of genetic diseases such as sickle cell anemia and Tay-Sachs disease are carried this way. Only when a child receives the recessive gay gene from both parents—a one-in-four possibility even if both parents are carriers—does he or she get the disease.

If it seems like I’m comparing homosexuality to a genetic disease, it’s because from an evolutionary point of view it might as well be. Dead from Tay-Sachs disease or childless from homosexuality, either way, your genes don’t make it into the next generation. (Well, not quite, as I’ll explain later.)

In addition, a recessive gene can sometimes still have an effect on the organism that carries it, and sometimes that effect can be advantageous, which allows the evolutionary process to select for it.

The classic example of this sort of genetic behavior is sickle cell anemia. Carriers of the gene experience few sickle cell symptoms, but have a much greater resistance to malaria. Within the human population, especially in Africa, the advantages of malaria resistance apparently outweigh the comparatively rare disadvantage of sickle cell anemia, so the disease persists.

It’s possible to imagine that carrying the gay gene confers some advantage. My personal theory (with absolutely no supporting evidence whatsoever) is that the gay gene could really be the seductive empathy gene: Men who carry this gene have a better understanding of what women want in men, and therefore they are better at seducing women. This confers an evolutionary advantage because the seductions lead to pregnancies, and half the children also carry the seductive empathy gene.

However, if the woman is also a carrier of the seductive empathy gene, then there’s that one-in-four chance that the child will receive both copies of the gene. This will give him such a complete understanding of what women want in men that he wants it himself. Thus, gayness.

(I should emphasize at this point that I totally made up the seductive empathy gene as an example of why the persistence of the gay gene is not a contradiction of evolutionary theory. There’s no proof for it at all…although it would explain why so many women have gay friends…)

As with eye color, reality is probably more complicated than this.

If there is a gay gene, it seems unlikely that it is 100% effective. Even if it was recessive, perfect effectiveness would produce a simple pattern of inheritance that probably would have been detected by now. I think it’s more likely that the genetics of gayness produce a tendency toward gayness, an increase in the probability of gayness, perhaps even resulting in differing levels of gayness.

(In this case, the gay gene wouldn’t have to be recessive.)

It’s also possible that gayness is controlled by multiple genes. There are several possibilities for how this could work, and all of them could be present at the same time.

Gayness may require a combination of genes to produce a gay individual, so that individuals without all the genes will not be gay. Some hereditary diseases work this way.

Or the gayness genes could add: The more gay genes you inherit, the gayer you are. Other human traits, such as height or skin color, seem to work this way.

Or maybe gayness has several different possible genetic causes. The genetics of hairless cats works this way: There are three breeds, arising from three unrelated genetic mutations.

Or maybe gayness isn’t one thing. There could be different types of gayness caused by different genes. White fur on cats works this way: any one of three different genetic mechanisms can cause a cat to have white fur. It may be that variations in behavior of individuals within the gay community are tied to different genes. For example, male and female homosexuality may have unrelated genetic causes.

Our observations about the evolutionary possibility of a gay gene are further complicated by the fact that we’ve evolved intelligence and culture, both of which tend to mask our evolved behaviors, and both of which have resulted in spectacular changes in the human condition. We no longer live under the conditions in which we evolved, and the gay gene might have had very different effects on humans in a state of nature than it does in our modern world.

Gay people today often feel strong pressures from society to hide their sexual preferences. I can only imagine that these pressures would have been far stronger in the restricted and stiflingly small tribes wandering the African veldt. It’s possible that gays felt irresistible pressure to mate and reproduce like everyone else. Social culture could have overridden genetic preference.

(That’s not uncommon: Heterosexuals often engage in homosexual acts while they are imprisoned, and many heterosexual couples in wealthy societies choose not to have children.)

Finally, there is an argument that homosexual members of a group may contribute to the evolutionary fitness of other members through kinship selection.

As a matter of genetic math, our genetic relationship to our siblings is exactly the same (50% matching DNA) as our genetic relationship to our children. Given the evolutionary imperative to spread our genes into future generations, we can do so either by ensuring that our children live long enough to reproduce, or that our brothers and sisters do. Similarly, our nieces and nephews are the evolutionary equivalent of our grandchildren.

As a consequence of all this kinship, a gay individual in a family can effectively reproduce his genes in future generations by helping his family to survive and thrive. Since some of them would also carry the gay gene, it too would survive into future generations.

I should emphasize that the evidence for all of these hypotheses is slim at best, and it’s not like I’ve done a massive amount of research to write this. I just find it fascinating that an apparently anti-reproductive trait like gayness could evolve at all. I’ve had a draft of this post sitting around for years, and I’ve started to work on it every time something relevant has been in the news, but I kept rewriting it, and the story always got stale before I could finish. The title was always the same though.