TORONTO — Homosexual men and women are more likely to be left-handed than their heterosexual counterparts — a finding that indicates sexual orientation, like handedness, may be determined before birth — Canadian researchers said Thursday.
In a study published in the July issue of "PsychologyBulletin," researchers said that a meta-analysis of 20 studies involving more than 23,000 men and women had found the odds that gays and lesbians would be left-handed was 39 percent greater than heterosexual people. A meta-analysis is a statistical technique using the results of previous studies to reach a reliable conclusion.
The study found gay men were 34 percent more inclined toward left-handednesss than non-gay men while the chances were even greater among homosexual women, where they were 91 percent more likely to be left-handed than heterosexual women.
"This is one more piece of evidence that sexual orientation is at least partly determined in utero," said Ray Blanchard, head of the Clinical Sexology Program at the Centre for Addiction and Mental Health in Toronto, and one of the study's authors.
The results also suggest there may be at least be one cause of homosexuality common to both men and women, Blanchard noted, adding that most biological research on sexual orientation has suggested that lesbianism and homosexuality have different causes.
Although it has not been conclusively proved that handedness is determined prior to birth, fetuses typically indicate a preference for the right or left hand while still in the womb — by, for example, moving it more frequently or sucking on a thumb.
Mario Lalumiere, another of the study's authors, said the study's main contribution was to provide "a window" into the early developmental period of the fetus.
"It's quite all right if we look, for example, to social factors in adolescence to try to explain a person's sexual orientation. That's fine. But it will never be able to explain every single case of homosexuality because we know for sure now that there are causes of sexual orientation that are much earlier than that," he said.