A scientist with a brain scanner could figure out your sexual orientation based on the symmetry of your brain, new research from the Stockholm Brain Institute hints.

The findings support the notion that biological factors help determine sexual orientation and leave a specific neuroanatomical signature.

Using MRI scans of gay and straight men and women, the researchers found that people who liked women – heterosexual men and homosexual women – had larger right brain hemispheres, while people who liked men – heterosexual women and homosexual men – had symmetrical brains. As seen in the image, MRI and PET scans showed a similar pattern in two specific regions of the brain, the right and left amygdalas, which are thought to control fight-or-flight reactions.

"The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities," the researchers, led by Ivanka Savic, write in a paper that will be published in the Proceedings of the National Academy of Sciences tomorrow.

Scientists have long tried to determine if sexual orientation is biologically determined, and if so, how. This research has been contentious in and outside the gay community. At stake is whether homosexuality is a choice or biologically inevitable. The groundbreaking work in the field of biological difference came from Simon LeVay, a self-identified gay neuroscientist, who found similarities in the brains of straight women and gay men in the early 90s. LeVay has his detractors, but recent studies seem to back his early research.

Pink News, billed as Europe's largest gay news service, cut straight to the chase about the implications of recent studies that have found measurable differences in the biology of men and women with different sexual orientations.

"These studies imply that the brains of the gay men have functional similarities to those of a straight woman, and that homosexuality is not of a moral choice, but one of biological substrate," wrote Jane Rochstad Lim.

While the research is suggestive, it does not address how such brain differences come to be, although the researchers noted that there could be genetic, environmental and/or sex hormonal factors.

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The Gay Bomb Lives!

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Image: Composite magnetic resonance images of the brains of 90 homosexual and heterosexual men and women.