Nature or nurture? Is sexual orientation something people are born with or a matter of choice? Canadian scientists say that genetics do play a role and have reported their findings in the Archives of Sexual Behavior. Carried out by Dr. Sandra Witelson, a neuroscientist at McMaster University, the study used Magnetic Resonance Imaging (MRI) to examine the brains of healthy, right-handed, 18-35 year-old homosexual and heterosexual men.
The new study is in some ways a sequel to Witelson's earlier work that demonstrated there are a higher proportion of left-handers in the homosexual population than in the general population - a result replicated in subsequent studies. Handedness is a sign of how the brain is organized to represent different aspects of intelligence. Language, for example, is usually on the left - music on the right.
In other research, Witelson and research associate Debra Kigar, had found that left-handers have a larger region of the posterior corpus callosum - the thick band of nerve fibers connecting the two hemispheres of the brain - than right-handers. This raised the hypothesis for the current study - whether the anatomy of the brain of the sub-group of right-handed homosexual men is similar to that of left-handers.
Witelson and her co-researchers found that the posterior part of the corpus callosum is larger in homosexual than heterosexual men. According to Witelson, the size of the corpus callosum is largely inherited, suggesting a genetic factor in 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," she said.
Witelson says the new study is far from definitive, but the new findings could prove to be an valuable piece of information for other researchers. Witelson also carried out further analyses that included the size of the corpus callosum and test scores scores on language, visual spatial and finger dexterity tests. "By using all these variables, we were able to predict sexual orientation in 95 per cent of the cases," she said.
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More Homosexuals Left-Handed
Source: McMaster University
