Sexual orientation linked to handedness

Palm readers take note: A team of Canadian psychologists suggests that part of understanding sexual orientation may be close at hand. The clue isn’t in the bend of the love line or length of the ring finger. It’s in which hand you present to the palmist.

The psychologists combined the results of 20 previous studies, both published and unpublished, comparing rates of right-handedness in a total of 23,410 homosexual and heterosexual men and women.

The researchers report in the July Psychological Bulletin that, overall, homosexual adults in the studies were 39 percent more likely than heterosexuals to use their left hand for more activities. The rate was even higher among lesbians, who were nearly twice as likely as heterosexuals to be left- or mixed-handed.

Because hand preference likely has a prenatal origin, the analysis supports the idea that sexual orientation also has early neurobiological roots, says Kenneth J. Zucker of the Centre for Addiction and Mental Health in Toronto and the University of Toronto, an author of the study. The analysis is notable, too, because it reveals a correlate of homosexuality—in this case handedness—that’s common to both gay men and lesbians. Many hypotheses posit separate explanations for the sexual preference of each group.

Although the team found a significant correlation, the size of the effect was small, prompting some researchers to downplay the sexuality-southpaw link. The team is “drawing a weak connection between one poorly understood phenomenon and another poorly understood phenomenon,” says Simon LeVay, a Los Angeles neuroscientist who in 1991 discovered structural differences in brain anatomy between gay and heterosexual men.

A widely reported paper in the March 30 Nature by a research team led by psychologist S. Marc Breedlove at the University of California, Berkeley suggested a link between a similar prenatal event and sexual orientation. The team found that lesbians had, on average, a longer ring finger than index finger, a pattern more often found in men and influenced by prenatal androgen exposure. The same study determined that homosexual men were more likely than heterosexual men to have several older brothers, whose gestation increases androgen levels in the uterine environment for subsequent births.

The authors of the new metanalysis argue against this androgen-exposure hypothesis. Although men are slightly better represented among left-handers than women, the researchers argue that high levels of androgens have not been linked to left-handedness in boys and are actually associated with right-handedness in girls. Instead, the researchers suggest the connection between handedness and sexual orientation may indicate developmental instability—the degree to which a fetus is exposed to environmental stressors, such as pollutants—or genetic mutations.

It’s too early to make such a conclusion, says J. Michael Bailey, a psychologist at Northwestern University in Evanston, Ill. “This is really the first finding that would point in that direction, and that’s not the only direction to point,” he says.

Breedlove agrees, accusing the Canadians of fatal vagueness. “What cellular process is unstable here?” he asks. “Cell death, neurogenesis, synapse elimination?” Because the theory carries such explosive political implications—homosexuality as “defect”—Breedlove finds its shortcomings disquieting.

Zucker readily acknowledges that an as-yet-unidentified variable may be responsible for the connection found by his team. “I’m satisfied with the fact that the metanalysis says there really is an empirical phenomenon to be explained,” he says, “and in research in sexual orientation, that’s an advance.”

From the Nature Index

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