There is a vivid debate if homosexuality is a well-defined genetic trait, like the skin, hair and eye color, or a matter of choice. But many human traits fall along a continuum, like the height, and the common concept that everyone is strictly either "gay" or "straight", could be misleading. Studies reached the same conclusion when it comes to homosexuality, and less than 10 % of the people score as "pure" heterosexual or homosexual and on average, females are more inclined than men are towards a homosexual orientation. This would mean that sexual orientation could be linked to flexibility or "choice" the person has in expressing that orientation, as it forms a continuum.
Another recent research showed that one third of the American football players considering themselves straight had some kind of homosexual inclination. Now, a Canadian research comes with a genetic (or hormonal?) proof that makes men prone to homosexuality. The team investigated - using structural Magnetic Resonance Imaging (MRI) - the brains of healthy, right-handed homosexual and heterosexual male subjects, aged 18 to 35. The same team had showed 10 years ago that there is a higher percentage of left-handed individuals amongst homosexuals than in the overall population, fact strengthened by subsequent researches.
Handedness is linked to the brain organization. Language and logical thinking are located normally in the left brain hemisphere, while music and affective intelligence in the right hemisphere. The new MRI pictures revealed that the left-handed individuals had a larger posterior corpus callosum (a brain part made of axons (nerve fibers) that connects the two brain hemispheres) than right-handed men had.
The researchers then checked if the brain anatomy of right-handed homosexuals resembled that of the left-handed ones. The posterior corpus callosum was larger in homosexual than heterosexual men. "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," said 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.
"While this is not a litmus test for sexual orientation, 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.", she said.
An analysis based on the size of the corpus callosum, and test scores scores on language, visual spatial and finger dexterity could "predict sexual orientation in 95 % of the cases", said Witelson.
Still, the direct genetic factor is not the only aspect that could explain this difference. In women, the corpus callosum is much bigger than in men. That's why women have their brain functions more distributed, while men have a more "asymmetrical" brain, with more specialized areas and, for example, a stroke on the left hemisphere, which can induce speech loss, it is more devastating for men. This means that homosexuals have more "womenized" brains (as found also by tests exploring their behavioral skills), fact that could be explained by hormonal ratios and balances in the womb. Indeed, this imbalance could have a genetic reason, but not necessarily.