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Gays and lesbians are more likely to be partly or completely left-handed, a Canadian study finds.

The study of 23,410 heterosexual and homosexual men and women showed that, overall, 26 per cent of homosexuals were partly or completely left-handed compared with 20 per cent of heterosexuals.

Lesbians, however, had much greater odds of being left-handed than gay men, according to the study.

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The authors say the differences between homosexuals and heterosexuals are large enough to suggest that at least one ingredient of sexual orientation is biological -- but it's not the whole story.

"I would not use a hand preference to determine whether or not to date a person," said Dr. Kenneth Zucker, head of the child and adolescent gender-identity clinic at Toronto's Centre for Addiction and Mental Health and one of the study's authors. "You would be wrong more often than right."

To arrive at their conclusions, the authors combined and analyzed the results from 29 past studies on sexual orientation and handedness. Their analysis is published today in the Psychological Bulletin, a journal published by the American Psychological Association.

The study validates a small study done a decade ago by McMaster University neuroscientist Dr. Sandra Witelson and colleague Cheryl McCormick.

Dr. Witelson's study inspired Dr. Zucker to review and conduct similar research. He bet his colleagues and co-authors, Ray Blanchard and Martin Lalumiere, both of the addiction centre and the University of Toronto, that if they reviewed all the research they wouldn't find a correlation between handedness and sexual orientation.

Today, he admits he was wrong.

Left-handedness is seen as an important indicator of a biological basis for sexual orientation because it's unlikely to be learned or copied.

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While the study released today confirms that the relationship between handedness and sexual preference is statistically significant, it doesn't weigh in on the question of how left- and right-handedness is decided in the first place. According to the authors, there are plenty of theories: It may be influenced by genetics, exposure to sex hormones as a fetus, changes in the mother's immune system or even how many stresses a pregnant mother withstands.

Still, the study's confirmation of a biological basis for sexual orientation sounded only vaguely convincing to many of the coffee-drinking crowd on a sun-drenched morning in the heart of Toronto's gay community yesterday.

Jay Matthews was the only left-handed one among three friends sitting at a Church Street café, but cohort Raymond Proulx said he was forced out of left-handedness as a child, and as a result is mixed-handed today.

Still, he's not sure what it all means.

"What's the point?" Mr. Proulx said yesterday. "What are we going to do with it, this knowledge?"

For Michael Bailey, an associate professor of psychology at Northwestern University in Illinois who studies the genetic basis of sexual orientation, the answer is simple.

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"It's about gaining fundamental knowledge about human nature," he said, "why humans are the way they are."

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