Are people born gay, or do they become that way as a result of childhood influences? The evidence seems to suggest that biology plays a crucial role in sexual orientation. While traditional psychoanalytic theory suggested that homosexuality could be the result of a child’s early interactions with both parents, there is little empirical evidence to support this position. It seems likely that this idea will go the way of similar ideas in psychoanalysis such as the “refrigerator mother” theory of autism, and the “double bind” theory of schizophrenia – that is into the dustbin of the history of ideas, replaced by much more robust biological theories based on scientific evidence.

The biology is by no means complete, but we do have at least two intriguing and compelling pieces of the puzzle.

The first is twin studies, which investigate patterns of inheritance. If you are a gay man, the chances your identical twin is also gay is around 50%. If he’s a non identical (fraternal) twin it’s around 22%, while an unrelated male’s chances of being gay are 2-4% (Not even the staunchest gay advocates believe Kinsey’s 10% these days!). If such studies are true then there are two conclusions you can draw.

Firstly, genetics plays a strong role in a person’s sexual orientation. The gradient from most to least genetically alike is paralleled by a decreasing probability two people share the trait of homosexuality. The second is that genetics is only part of the story.

Twin studies raise an obvious question. If homosexuality is a genetically determined trait (at least partially), why would such a genetic pattern survive the rigors of natural selection? Does this make evolutionary sense? Could such a genetic pattern confer a survival advantage in terms of Darwinian “fitness”? My answer is yes. Read on.

The second piece of the puzzle is birth order studies.

Gay men are more likely to be younger brothers than older brothers. It turns out that for each older brother a man has, his chances of being gay increase by around 30%.

Now could this be because younger brothers have a different roles and life experiences in families from older brothers? Could this hint at sexual abuse of the younger by the older being a cause - the hoary old chestnut of the “seduction theory” of homosexuality with big brother in the frame? It turns out this is not the case. Recent research indicates that the effect is preserved even when adoption results in a different age order in the adoptive family. A younger brother, for example, retains his increased probability of being gay, even if he is adopted out to a family where he becomes the oldest brother. The effect is biological, and related to the birth order of sons of the biological mother, not of the role in the social family.

So how could such a pattern be adaptive in an evolutionary sense? Surely a gay man is less likely to have his genes pass on to successive generations, and thus the “homosexuality genome” is likely to die out? Not necessarily.

Birth order studies suggest that the genetic pattern “for” homosexuality is more likely to be expressed if there is already an older brother around. In other words, both brothers may be equally likely to carry the pattern, but it is only expressed as a homosexual orientation in the younger. Could this be a way of biologically signaling a cooperative “division of reproductive labour” between brothers?

Reproduction is much more biologically expensive for females than males. A single male can easily service the reproductive needs of multiple females, which means that in many species there is competition between males for the reproductive “spaces” the females hold. Now, such competition is itself expensive. A pair of brothers who are fighting each other for the same reproductive opportunities has less energy to put into ensuring the survival of the offspring.

Now suppose there was some biological mechanism by which you could knock out your closest competition for female mates (your younger brothers), and yet preserve the advantage of having close male kin to assist in the care and protection of your offspring. It would definitely be a survival advantage for an older brother to carry the genes for such a mechanism.

But does the younger brother get a raw deal (reproductively) from such an arrangement? Not as raw as it first seems. Nieces and nephews carry 25% of your genes, the same percentage as your grandchildren. Now, you could fight your older brother for your own reproductive opportunity and get 50% into each of the next generation, but it may be a better strategy to concede and be satisfied with 25%, especially if it means more nieces and nephews than you’d otherwise have. Your older brother is always likely to be bigger and stronger than you until he passes his peak, at which stage the hierarchy reverses, but it’s better to reproduce before, rather than after the peak. As well as that, you save the energy (and risk of injury) you’d waste fighting and can both put it into raising the next generation.

Of course this only explains the negative (non heterosexuality) rather than the positive (homosexual attraction), but that, I suspect, is another story.

So did my brother turn me gay? I’d be really interested in people’s opinions on this, especially from those who know a bit more about evolutionary psychology than I do.

http://www.worldpolicy.org/globalrights/sexorient/twins.html
http://www.sciam.com/article.cfm?articleID=0005A6D3-7ADC-14A0-B6C483414B7F4945

Who cares, you pervert.

Leigh, given yours and others concern about homosexuals it should be very important to you to understand what causes it. My guess is that you'd rather not know because knowing will make it hard to carry on with cries of "pervert/abomination/unnatural/etc". Most of you seem to manage to do so in the face of a large body of evidence that suggests that a homosexual orientation is mostly genetic.

My view is that if it is primarily genetic then trying to force gays into a straight pattern just causes a whole lot of unnecessary grief and genuine perversion - the creeps who hang around tiolet blocks because they don't feel they can be open about being gay at home or work, the married guys carrying on gay affairs on the side because they've tried to be straight and not succeeded.

The gays themselves, their heteropartners and others around them are subject to a bunch of unnecessary grief because of social pressures that benefit nobody. Where is the win for any of us if a small number of gays live as hetero's (mostly not happily so)?

I'd much rather see gays finding satisfying relationships with other gays than feeling forced into unhappy relationships with heterosexuals - what about you? Do you have a daughter you like to have married to a guy who'd rather be with another man?

Snout, I saw a doco a couple of months ago covering much of the material you mentioned. It was very interesting. I look forward to seeing if the issues you raised can be discussed.

R0bert

Oh good one Leigh - that comment confirms my earlier impressions of your bigoted intolerance of those who are different from you.

And you called my post on Boazy's latest loony Islamophobic thread "inconsequential"! It took you less than 24 hours to outdo me in the inconsequentiality stakes.