Big Question: Is there a "gay gene"?

Curiosity contributor Susan Sherwood took a look at the question of genetics and homosexuality to see whether our wiring had a say in our preferences.

In December 2011, actress Cynthia Nixon told the New York Times Magazine that, after being in both straight and gay relationships, she chose to be a lesbian. Some, who believe that homosexuality is inborn, thought she may be misguided and that her position threatened their advocacy. Others cited her statement as proof that being gay is a choice and doesn’t deserve protective legislation comparable to race and sex discrimination laws.

What does science say about it? An early study in 1993 from the National Institutes of Health investigated gay men and their families. The results indicated that homosexuality had a genetic link through the maternal line. Although they don't know the precise gene, researchers believe they've located the relevant chromosome. A related study on twins out of Northwestern University concluded that, if one identical twin was gay, the other had a 50 percent chance of being homosexual. With fraternal twins, the likelihood was only 20 percent [source: Discover Magazine].

If identical twins, with identical genes, don’t always have 100 percent identical sexual orientation, what other factors could be involved? Environment? Not according to a University of Oklahoma study of male infants who had been surgically altered to be females soon after birth, due to genital abnormalities. Though raised as females, unaware of their birth realities, all of the men were, as adults, attracted to women.

What about the gestational environment? A University of Toronto study found that males were more likely to be gay if they had older brothers. Not older brothers who were gay, just older brothers. Replication of this research indicated that the home environment was not a factor (e.g., being teased by older siblings). Furthermore, the effects were seen even if the younger sibling had been adopted. Perhaps, then, the complete genetic components have just not been fully identified.

Do the conclusions hold true for lesbians, as well? Not necessarily, according to a study from the University of Utah. Women’s sexuality appears to be more fluid; they are more likely than men to alter their sexual preferences. How do researchers explain this difference? Is it genetic? Neurobiological? Contextual? The door is still open.