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Study: gays’ brain symmetry resembles other sex

June 16, 2008
Courtesy PNAS
and World Science staff

Re­search­ers have found that gay peo­ple’s brains re­sem­ble those of the op­po­site sex in some ways, in­clud­ing the ex­tent to which the brain’s sides are sym­met­ric.

Amygdala activity in (top to bot­tom rows) he­ter­o­sex­ual men, he­ter­o­sex­ual women, ho­mo­sex­ual men and ho­mo­sex­ual women. (Im­age cour­tesy PNAS)


Some psy­cho­log­i­cal tests have shown dif­fer­ences be­tween men and wom­en in how much they use each of the brain’s hemi­spheres, or op­po­s­ite sides, in ver­bal tasks. 

Oth­er re­search has hinted that ho­mo­sex­u­als ex­hib­it the ten­den­cies of the op­po­site sex in brain be­hav­ior un­re­lat­ed to sex­u­al ac­ti­vity.

In the new stu­dy, Ivanka Sa­vic of the Ka­r­o­lin­ska In­sti­tute in Stock­holm and col­league per­ Lind­ström found that the brains of he­tero­sex­u­al men and homo­sex­u­al wom­en are slightly asym­met­ric: the right hem­i­sphere is larg­er than the left. 

But the brains of gay men and straight wom­en are not, they found.

The scans al­so re­vealed dif­fer­ences in the way the amyg­dala—a brain struc­ture im­por­tant for emo­tion­al learn­ing—is con­nect­ed to the rest of the brain, the re­search­ers re­ported. In these con­nec­tions, they said, gay men re­sem­ble straight wom­en, and les­bians straight men. 

A pos­si­ble ex­plana­t­ion is that the amyg­da­la in the lat­ter group is wired for a great­er “fight-or-flight” re­sponse, said the re­search­ers, who an­a­lyzed the brains of 90 sub­jects. 

The sci­en­tists used the scan­ning meth­od known as mag­net­ic res­o­nance im­ag­ing or MRI to as­sess brain size, as well as ad­di­tion­al da­ta from a sep­a­rate scan­ning meth­od called pos­i­tron emis­sion to­mog­ra­phy or PET. The find­ings are pub­lished in this week’s early on­line edi­tion of the re­search jour­nal Pro­ceed­ings of the Na­tio­n­al Aca­de­my of Sci­en­ces.


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Researchers have found that gay people’s brains resemble those of the opposite sex in some ways, including the extent to which the brain’s sides are symmetric. Some psychological tests have shown differences between men and women in how much they use the brain’s hemispheres, or different sides, in verbal tasks. Other research has hinted that homosexuals exhibit the tendencies of the opposite sex in brain behavior unrelated to sexual activity. In the new study, Ivanka Savic of the Karolinska Institute in Stockholm and colleague Per Lindström found that the brains of heterosexual men and homosexual women are slightly asymmetric—the right hemisphere is larger than the left. But the brains of gay men and straight women are not, they found. The scans also revealed differences in the way the amygdala—a brain structure important for emotional learning—is connected to the rest of the brain, the researchers reported. In these connections, they said, gay men resemble straight women, and lesbians straight men. A possible explanation is that the amygdala in the latter group is wired for a greater “fight-or-flight” response, said the researchers, who analyzed the brains of 90 subjects. The scientists used the scanning method known as magnetic resonance imaging or MRI to assess brain size, as well as additional data from a separate scanning method called positron emission tomography or PET. The findings are published in this week’s early online edition of the research journal pnas.