(Disclaimer: This post contains no political, ethical, moral, or religious conclusions. Any such insights the reader draws from it are their own. This post is simply my understanding of the current science.)
The start of this conversation was my assertion that sexual attraction is hard-coded into human physiology, and that culture/socialization may encourage or discourage our acting on that attraction, but culture cannot create a sexual orientation where none exists in the biology. My analogy for this is diet. A culture can influence what you eat, and how you eat, but only within the limits of a maximum possibility; it can't make you an herbivore. You just don't have the biology for it. JN's strongly held belief is that culture can in fact create homosexual attraction.
As a primary source, JN provided a link to this Columbia University paper which ruled out simple genetic and hormonal models of homosexual attraction, and posited that there must be socialization components to this behavior to cover the explanatory "gap" created by the genetic/hormonal explanations. Their primary reason for believing this was in opposite-sex twins (one boy, one girl) the boy was more likely to express homosexual attraction as an adolescent if he had no older brother, but showed the same odds of expressing homosexual attraction as anyone else if he did have an older brother. The presence of an older brother obviously cannot effect uterine environment or genetics, so the conclusion was that the older brother provided a social role model that guided the boy-twin away from homosexual attraction.
In opposition to this paper, Wikipedia provides a lengthy list of physiological markers which are different between gay and straight members of both genders. There are differences in brain structures, finger lengths, startle responses, handedness, hair-whorl direction, and so forth between gay and straight populations, and in many of these categories the homosexual shows characteristics associated with a heterosexual of the opposite gender. Put simply, there is no way culture or socialization can change the length of your fingers the shape of your cerebral lobes, especially so when this markers are present prior to birth. It is "unpossible".
So where does that leave us? The Columbia paper rules out a simple genetic explanation of homosexual attraction, and the existence of physiological markers rules out culture and socialization.
I believe that the Columbia paper is mistaken in two respects. Firstly, they only measure self-reported attraction, not biological markers. And secondly, they used the wrong model of how genetics work. I don't blame for that though, as the paper was written in 2001 and we have learned a lot about genomics in the last decade.
Firstly, let's dispense with the self-reporting. We've known since the Kinsey Study that as much as 10% of the population may engage in homosexual activity at least once during their adult years. Sexuality isn't an on/off switch between gay and straight; there's a range between the two, with individuals reporting a varying degree of bisexuality. It makes perfect sense that if a person is biologically bisexual (but not strongly so, maybe a 1 or 2 on the Kinsey Scale) that culture or socialization can influence whether they explore those feelings. That would explain entirely how birth order could affect self-reported feelings of attraction.
As for the genetic model, we have learned in the last decade that the old Mendel model of discrete genes is false. Craig Venter (who won the Human Genome Project prize by sequencing his own DNA, along with several others) had this to say about his own genetics:
"I found out that I have a high probability of having blue eyes," the blue-eyed Venter said in a telephone interview.Craig Venter was born with blue eyes. They never changed to any other color at any point during his life. Culture and socialization had nothing to do with it, any more than it did the shape of his nose. But his DNA doesn't say for certain his eyes would be blue either, only that it was a probability. And this is the difference between genotype and phenotype.
"You can't even tell with 100 percent accuracy if I would have blue eyes, looking at my genetic code," he laughed. "We all thought that would be simple."
Your DNA is your genotype. It says what's probable, but doesn't lock in hard-coded certainties in all respects. All it does it set the beginning state of an incredibly complex and self-organizing dance of molecules that turns two sex cells into a zygote and eventually a baby. But this process is not a fancy clock following set steps, it's a big messy chemistry bath subject to hormonal signals, uterine environment, and pure random chance.
By the time you're born though, the roulette wheels have all stopped spinning. Your cerebral lobes are either symmetrical, or not. Your index and ring fingers are either the same length, or not. There's no going back, and there's no socialization that will change them. To borrow an analogy from quantum physics, DNA is the quantum state of probability that existed before your conception, and your phenotype is the observed result - and thereafter fixed. The only thing society can do is encourage or discourage you from acting on your phenotype's existing and hard-coded predisposition to various behaviors.
Sidebar: What about the Greeks?
This section is primarily editorial in nature, and isn't based on much science (collecting firm data from 2,500 years ago just isn't possible). It addresses the point some people raise about how some societies (such as the Ancient Greeks) saw widespread homosexual activity, far greater than the 10% of the population found by the Kinsey Study to be at least partially bisexual. The argument here is that culture can in fact create homosexual attraction, despite everything I said above. I mean, haven't you read The Symposium by Plato?
Yes, I have. And I don't find Plato to be a trustworthy narrator. Rather than high-minded ideals of love and attraction, I'm reminded more of American prisons and NAMBLA apologists.
Among gays, there is a distribution of individuals who prefer top, bottom, and versatile positions during sex. There's no scientific consensus on what exactly the distribution is, or how culture may effect it, but the existence of these preferences are beyond dispute. I would call them "common knowledge" among the gay community, and I believe my gay friends when they tell me they have such preferences.
No such distribution is observed in the Ancient Greek tradition of paiderasteia. The older male is always the top, and the younger male always the bottom. If these were actual homosexual relationships the reverse would be true at least half the time. The practice of paiderasteia (both in Ancient Greece and in primitive tribes in Papa New Guinea, who have been studied by modern sociologists) is only consistent in my mind with institutionalized sexual abuse. The young men studied in primitive tribes show biological markers of abuse too, even where their culture say it's "Okay" for older men to do those things. They show stress markers, and are not engaging in the practice joyfully. If we had a time machine, I'd bet $1,000 we'd find the same among the Ancient Greeks.
The male sex drive is very strong, and if access to females is restricted (whether by the social rules of Athens, or because the male is locked up in prison, or because he's a shepherd alone with his sheep for weeks at a time), then it will find outlets by other means. The Ancient Greeks are only unique, in my mind, with the lengths they went to to romanticize and justify the practice.