Washington, Nov 8: Canadian scientists have shed more light on the debate whether sexual orientation is a genetic trait or a matter of choice.
Dr. Sandra Witelson, a neuroscientist in the Michael G. DeGroote School of Medicine at McMaster University, and colleagues at Sunnybrook Health Sciences Centre in Toronto have uncovered new evidence, which shows that genetics has a role to play in determining whether an individual is homosexual or heterosexual.
For the study, the research team studied the brains of healthy, right-handed, 18- to 35-year-old homosexual and heterosexual men using structural Magnetic Resonance Imaging (MRI).
About 10 years ago, Witelson and Dr. Cheryl McCormick, demonstrated that there is a higher proportion of left-handers in the homosexual population than in the general population – a result replicated in subsequent studies that is now accepted as fact.
Handedness is a sign of how the brain is organized to represent different aspects of intelligence. For example, language is usually on the left, while 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 fibres 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.
Analysis showed that the posterior part of the corpus callosum is larger in homosexual than heterosexual men.
According to researchers, 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," Witelson said.
She added that this finding could prove to be one additional valuable piece of information for physicians and individuals who are trying to determine their sexual orientation.
“Sometimes people aren"t sure of their sexual orientation."
The researchers also undertook a correlational analysis, which included size of the corpus callosum, and test 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.
The study is published in the on-line edition of the Archives of Sexual Behaviour.