The researchers found an important brain structure -- the posterior region of the corpus callosum -- is larger in homosexual than in heterosexual men. The brain structure consists of a thick band of nerves that connect the left and right hemispheres of the brain.

Dr. Sandra Witelson of McMaster University, with colleagues at the Sunnybrook Health Sciences Center in Toronto, studied the brains of healthy, right-handed, 18- to 35-year-old homosexual and heterosexual men.

Witelson's team had observed the same structural difference in a prior study of left-handed men and wanted to test the hypothesis that the brain anatomy of right-handed homosexual men is similar to that of left-handers.

The scientists said the findings are consistent with that hypothesis, as well as the theory that genetic factors are a major determinant of sexual orientation.

"Our results do not mean that heredity is destiny, but they do indicate that environment is not the only player in the field," said Witelson.

The study appears in online edition of the journal Archives of Sexual Behavior ahead of print publication.