Striking similarities between the brains of gay men and heterosexual women have been discovered by neuroscientists, offering fresh evidence that sexual orientation is hardwired into neural circuitry.
Scans reveal that homosexual men and heterosexual women have symmetrical brains, with the right and left hemispheres almost exactly the same size. Lesbians and heterosexual men have asymmetrical brains, with the right hemisphere significantly larger than the left.
Scientists at the Stockholm Brain Institute in Sweden also found that certain brain circuits linked to emotional responses were the same in gay men and heterosexual women.
The findings, published today in the US journal Proceedings of the National Academy of Sciences, suggest the biological factors that influence sexual orientation, such as exposure to testosterone in the womb, may also shape the brain’s anatomy. The study, led by neurobiologist Ivanka Savic, builds on previous research that has identified differences in spatial and verbal abilities related to sex and sexual orientation. Tests have found gay men and heterosexual women fare better at certain language tasks, while heterosexual men and lesbians tend to have better spatial awareness.
Savic and her colleague Per Linstrom took MRI brain scans of 90 volunteers, who were divided into four age-matched groups according to whether they were male or female, heterosexual or homosexual.
The scans showed the right-hand side of the brain in heterosexual men was typically 2 percent larger than the left. Lesbians showed a similar asymmetry, with the right side of the brain 1 percent larger than the left. Scans on homosexual men and heterosexual women revealed both sides were the same size.
The results could explain a study led by Qazi Rahman at Queen Mary, University of London, which found gay men and heterosexual women share a poor sense of direction compared with heterosexual men, and are more likely to navigate using landmarks alone.
The right hand side of the brain dominates spatial capabilities, so may be slightly more developed in heterosexual men and lesbians. An earlier study by the same team found gay men and heterosexual women outperformed lesbians and heterosexual men at tasks designed to test verbal fluency.
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