As gay couples race to the altar in California this week, scientists may have found an answer to the so-called gay paradox. Studies suggest that homosexuality is at least partly genetic. And although homosexuals have far fewer children than heterosexuals, so-called gay genes apparently survive in the population. A new study bolsters support for an intriguing idea: These same genes may increase fertility in women.
Despite some tantalizing leads over the past 2 decades, researchers have yet to isolate any genes directly linked to homosexuality. Nevertheless, a number of studies have shown that male homosexuals have more gay male relatives on their maternal lines than on their paternal lines, leading some scientists to suggest that gay genes might be found on the X chromosome. And in 2004, a team led by evolutionary psychologist Andrea Camperio Ciani of the University of Padua in Italy reported that women related to gay men had more children than women related to heterosexual men. The differences were striking: The mothers of gay men, for example, had an average of 2.7 children, compared with 2.3 children for the mothers of heterosexual men. A similar trend held for maternal aunts.
In new work, reported online this week in PLoS ONE, Camperio Ciani and his colleagues used mathematical modeling to see what kinds of genetic scenarios could explain these results. The team looked at more than two dozen possibilities, such as the number of "gay genes" (one or two), how much of a reproductive advantage the genes provided, and whether the genes were located on the X chromosome or other, nonsex (autosomal) chromosomes. The model that best explained the data consisted of two "gay genes," with at least one on the X chromosome. These genes increased the fertility of women but decreased it in men--a phenomenon previously studied in insects and mammals called "sexual antagonism."
Camperio Ciani's team suggests that these gay genes may actually increase how attracted both men and women are to men rather than making gay men more "feminine," as some researchers had earlier proposed. Although this is bad for male fertility, it is good for female fertility and allows such genes to survive at low but stable rates in a population, the authors say.
Dean Hamer, a behavioral geneticist at the National Cancer Institute in Bethesda, Maryland, who pioneered the search for gay genes, calls the study "an elegant mathematical analysis." He adds that the team has come up with a "simple solution" to the Darwinian paradox posed by homosexuality: "What is a 'gay gene' in a man is a 'superstraight gene' in a woman," he says.