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Time for a thought experiment: Are straight people born that way? When I put the question to a number of sexology colleagues, they thought it a good question -- indeed, a hard question.

To answer it, we have to start with a more fundamental question: What do we mean when we say someone is "straight"? At the most basic level, we seem to be imagining female bodies that are specifically sexually aroused by male bodies, and vice versa.

Laboratory studies such as those conducted by Michael Bailey of Northwestern University and Meredith Chivers of Queens University suggest that, while such people probably do exist -- at least in North America, where many sexologists have focused their attentions -- it's not uncommon for straight-identified people to be at least a little aroused by the idea of same-sex relations.

The media has tended to broadcast the news that gay-identified men and straight-identified men have quite discernible arousal patterns when they are shown various kinds of sexual stimuli. And that's true. But if you look closely at the data, you'll see that most straight-identified men do tend to show a little bit of arousal across sex categories (as do gay-identified men).

Developmental studies by researchers like the University of Utah's Lisa Diamond suggest that women may demonstrate a fair bit of flexibility over the course of their lives in terms of their sexual attractions, relations, and identities, and laboratory studies by people like Chivers and Bailey seem to back up the hypothesis that females are, on average, less rigidly oriented when it comes to sex. At least when you measure their vaginal response, straight-identified women can be aroused by a wide variety of sexual stimuli.

Nevertheless, let's just say, for purposes of argument, that we're willing to accept the existence of some straight females and males as defined above, even if they are not always perfectly straight arrows in the lab. Were they born that way?

When I put the question to Eric Vilain, a pediatric geneticist at the University of California, Los Angeles, who studies sexual development, he didn't like the way I phrased it, because babies don't get aroused by particular sexual scenarios.

"If we are literal about 'born this way,' we have to say that humans do not appear to be born with particular arousal patterns -- babies aren't gay or straight," Vilain told me. He suggested we think about the question a different way: Do we see evidence that, from a very early age, children exhibit behavior patterns that appear highly predictive of future sexual orientation?

In other words, do children give us clues about whether they're going to ultimately be sexually attracted to males, females, or both? To a certain extent, yes. That's why plenty of gay and lesbian adults can point to childhood clues that they were "born this way." Most straight people could do the same, although typically no one asks straights when they knew they were straight. Behavioral patterns in childhood do show some correlation with adult sexual orientation.

Vilain points, for example, to the evidence from girls born with a disorder called congenital adrenal hyperplasia (CAH), including that gathered by the University of Cambridge psychologist Melissa Hines. CAH results in naturally increased levels of androgens (a "masculinizing" type of hormone), and females with CAH show relatively high early interest in male-typical childhood activities as well as relatively high rates of bisexuality and lesbianism as adults.

Gendered behaviors are linked closely enough to sexual orientation cross-culturally that various cultures have developed third-gender categories that "normalize" a homosexually-oriented person. For instance, in Samoa, boys who are very feminine as young children are understood to be destined for attraction to males. They are relabeled "fa'afafine" -- meaning they will live "in the manner of a woman." Without changing their bodies, the fa'afafine are raised like girls and then live as women, and take straight men as their sex partners.

Sexologists call this kind of phenomenon "homosexual transgenderism" and suggest it is fairly common around the world. Sometimes "homosexual transgenderism" is enacted via a humane cultural system, as in Samoa, and sometimes via a phenomenally oppressive one, as in Iran, where feminine homosexual men have been given the choice of transsexualism or death.

Regardless of the cultural system, social pressure to appear straight seems to be fairly intense cross-culturally. Indeed, one is inclined to wonder, if being straight is just natural, why does it require quite so much policing?

Still, there does seem to be some biological evidence that at least some straight people probably were born strongly inclined that way.

Ray Blanchard of the University of Toronto has articulated the "fraternal birth order effect" (FBOE): The more older brothers a male has from the same biological mother, the more likely he is to be a gay adult. The theory is that the mother builds up an accumulating immune response to male fetuses, progressively dampening down masculinity of later-born male fetuses. That's just a theoretical explanation, although the FBOE itself is unequivocally real; it holds up in study after study across cultures. Blanchard has estimated that the 15 to 29 percent of gay men are gay by virtue of the FBOE. (The effect doesn't exist with women.)

While the FBOE is usually used to talk about the origins of male homosexuality, it could just as well be seen as suggesting that a particular womb environment is likely to produce babies who will grow up to be heterosexual men. In other words, the FBOE suggests that it is likely that many straight men were born inclined to be straight. Note this wouldn't be because of these straight men having been born with a "straight gene." They would be born inclined-straight following complex interactions of maternal and fetal genes.

Is there any evidence for "straight genes," other than the rather indirect evidence of the large number of people who identify as straight? Researchers have looked at sexual orientation among monozygotic twins -- that is, twins who are genetically identical -- versus dizygotic twins. Monozygotic twins are more likely than dizygotic to have the same sexual orientation (to both be straight or both be gay), but even they are not always matched. Bailey concludes that the data are "consistent with some genetic influence" for sexual orientation but that the data are "not overwhelming." He goes so far as to say "the evidence from twin studies for innateness of sexual orientation is pretty weak."

That said, Bailey does see some other evidence for an innate component to sexual orientation, at least in males. He points to cases where genetic males have been surgically and hormonally turned into girls in infancy, either because of childhood accidents that obliterated their penises or because they were born without penises and thus doctors subjected them to sex change. As adults, these folks are typically attracted to females. Says Bailey, "if you can't make a genetic male be attracted to other males by rearing him as a girl from early in life, how likely is any socialization theory of homosexuality or heterosexuality? I think not likely," at least for males.

Raymond Hames, a cultural anthropologist at the University of Nebraska, has been working with his students to survey the preponderance of homosexuality in various cultures. His team finds that, in more cultures than Americans might guess, same-sex encounters take the form of adult males obligating boys to sexually pleasure them. Many children don't appear to enjoy this; they do it because it is required as a kind of rite of passage. Importantly for this discussion, these kinds of cultures don't seem to produce many men who are attracted to men. In other words, these early same-sex experiences don't seem to "turn" the boys gay.

While it has been asserted by some that abuse at the hands of men might incline girls to be more likely to ultimately become lesbians, the evidence for this claim is weak. Boston Children's Hospital public health researcher Bryn Austin and her colleagues have documented that lesbian and bisexual women report having suffered higher rates of physical and sexual abuse in childhood and adolescence, a finding borne out by other teams' investigations. But we can't show any kind of clear causal link between the experience of childhood abuse (sexual or physical) and adult sexual orientation.

For abuse to push children toward straight or gay orientations they might otherwise not have had, it would have to be the case that children's sexual orientations can be shifted in direction, intentionally or unintentionally, and the truth is that the evidence for that is thin at best.

In short, we don't really know where human sexual orientations come from yet. What we do know is that the evidence we have that sexual orientation includes an innate component doesn't seem to point to the existence of simple "gay genes" and "straight genes." The best scientific argument we have for the innateness of straightness is that evolution obviously would favor it. (Yup: The strongest empirical rationale religious conservatives could use for the idea that straight people are born that way would come from a branch of science they generally disregard.)

Finally, a related question: Given that we don't know if they are really all "born that way," should straight-identified people be allowed to marry?

Personally, I think it makes sense to let straight-identified people marry, not because they were necessarily born that way, but because it seems silly, in this day and age, to get in the way of their desire to marry and/or to have sex with whatever consenting adults they wish. Given the challenges of attempting a lifetime partnership with a person who will be, on average, fundamentally sexually different from oneself, it seems the least we can do for straight people is to let them get married if they want.

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