Most scientists who study human sexuality agree that gay people are born that way. But that consensus raises an evolutionary puzzle: How do genes associated with homosexuality avoid being weeded out by Darwinian evolution?
Some gays and lesbians do reproduce, said Pennsylvania State University anthropologist and geneticist Mark Shriver, but not as much as straight people do. Even if a gene decreases people's fertility by 1 percent, it's going to be eliminated.
Scientists offered some possible answers to this mystery earlier this month at a Penn State symposium on the biological basis of sexual orientation. The seminar was planned weeks before the child sex-abuse scandal broke, and organizers said they never considered scrapping the program. How, they asked, could anyone fault them for talking about sex amid a scandal that centered on the failure to talk?
Not that the day lacked controversy and even anger. Several audience members said they were upset, or even "horrified" after Penn State's Shriver suggested that unraveling the biology of sexual orientation could lead to ways for parents to increase or decrease their odds of having a gay child.
Shriver made that forecast in response to a talk by University of Toronto psychiatry professor Ray Blanchard, who presented survey data showing that men with older brothers were more likely to be gay.
Blanchard, who treated patients at the university's Centre for Addiction and Mental Health, said he didn't think it was likely that parents would be rushing to alter a fetus' sexuality.
But antigay prejudice is still common in America. That was made clear last week in the responses to President Obama's promise to use foreign aid to discourage countries such as Afghanistan, Saudi Arabia, and Uganda from executing gays.
"Just when you thought Barack Obama couldn't get any more out of touch with America's values . . . this administration's war on traditional American values must stop," responded Republican presidential contender Rick Perry.
At the meeting, researchers assumed that science would encourage tolerance. That has been the long-term trend, said Blanchard.
For most of the 20th century, he said, the medical community wrongly said homosexuality was rooted in childhood experience. That came from Freud and also from the school of psychology known as behaviorism, pioneered by that famed rat researcher B.F. Skinner. The behaviorists thought homosexuality was a mental illness that could be cured, usually by giving electric shocks and other painful stimuli to try to create an aversion to homosexual thoughts.
The psychotherapists used a different technique and told patients it would work only if they really wanted to change, Blanchard said. "I'm sure it didn't change anybody." Nevertheless, both types of therapy were common practice into the 1970s.
Gradually, Freud and Skinner fell from prominence and people started thinking that DNA could influence sexual orientation. In the 1980s, a geneticist named Dean Hamer studied 40 pairs of brothers in which one or both were gay. He found a particular stretch of DNA on the X chromosome that was shared more often when both brothers were gay. When he published the results in 1991, it became known as "the gay gene," though Hamer has objected to this as an oversimplification.
Still, it immediately raised questions about natural selection, said Blanchard. "If there was a gene, then why isn't it flushed out of the population?" he said. "People still wrestle with it."
Studies that attempted to replicate Hamer's finding came up with conflicting results. Today, scientists suspect there are many genes with a small influence on sexual orientation.
The scientists said they had tended to focus more on men because they're easier to study. One of the other speakers, J. Michael Bailey, explained this in a provocatively titled talk: "What Is Sexual Orientation and Do Women Have One?"
Bailey, a psychologist at Northwestern University, started by explaining that you have to be very specific about the way you define sexual orientation because there are big differences between what people want, what they do, and what they will admit to doing.
Attempting to get at sexual orientation with a scientific approach, researchers have brought men and women into their labs, hooked their genitalia up to various devices, and tried to measure how aroused they got in response to films with a sexual content.
They learned that the majority of men respond strongly to images of either men or women and not to the other. Lesbians respond more strongly to seeing women, but straight women respond about the same to either sex.
It's not that scientists don't care about female sexuality, Blanchard said, but it's easier to classify men as gay or straight.
He was intrigued by the fact that some large surveys have shown that men who have older brothers were more likely to be gay, and that likelihood went up with the number of older brothers. It was as if male fetuses but not female ones left the uterus in a different state than they found it.
There are other conditions that follow this pattern, he said. In some cases, when a mother's blood type is incompatible with that of her fetus, she produces antibodies that can cause illness to a subsequent fetus.
In the case of male homosexuality, he said, the mother might build up antibodies to proteins that are coded in the Y chromosome, and therefore only expressed in male fetuses. Several of these are suspected to be active in the development of the male brain.
So in theory, antibodies remaining from a previous pregnancy could disable these male proteins in a fetus, thereby shaping the brain more along female lines.
The immune idea doesn't change the assumption that genes play a role. Many gay men have no older brothers, after all. And twin studies show that if one member of a pair of identical twins is gay, there's a 50 percent chance the other will be, too. For fraternal twins, that falls to 20 percent, a strong indication there's a genetic component to homosexuality.
Penn State's Shriver pointed out several ways that genes predisposing men to homosexuality could continue to thrive in the human pool. One possibility is that they made women more fecund, perhaps by increasing their sex drive. Another possibility, he said, is group selection - a form of natural selection that acts on the level of groups of organisms.
Scientists disagree on the importance of group selection in shaping humanity, but the basic idea is that if different tribes of humans are competing, some groups will thrive while others go extinct.
Perhaps diversity was good for group survival: Tribes of people that contained all breeders might have been less successful over prehistory than tribes that had almost all breeders except for a Leonardo da Vinci and a Michelangelo.
Or more generally, groups with greater genetic diversity might thrive and grow faster than more uniform groups, especially in variable environments.
Blanchard said he was not particularly worried that parents could someday abort their fetal Leonardos or rearrange their brains with immune therapy.
For one thing, he said, the most avidly homophobic people deny any link to biology. And speculative fears of nefarious technology shouldn't turn people against science. "What scientists do is to discover the truth," he said, "or model the truth as best they can."
Contact staff writer Faye Flam at 215-854-4977 or email@example.com. Read her "Planet of the Apes" blog at www.philly.com/evolution.