Swedish researchers have found that some physical attributes of the homosexual brain resemble those found in the opposite sex, according to an article published online (June 16) in the Proceedings of the National Academy of Sciences.

Some psychological tests have shown differences between men and women in the extent to which they employ the brain’s hemispheres in verbal tasks. Other research has hinted that homosexuals may exhibit the tendencies of the opposite sex in brain behavior unrelated to sexual activity.

Ivanka Savic and Per Lindström, of the Department of Clinical Neuroscience at the Karolinska Institute in Stockholm, Sweden, now report that the brains of heterosexual men and homosexual women are slightly asymmetric—the right hemisphere is larger than the left—and the brains of gay men and straight women are not.

Positron emission tomography (PET) scans taken by the researchers also show that in connectivity of the amygdala (which is important for emotional learning), lesbians resemble straight men, and gay men resemble straight women. The researchers analyzed the brains of 90 subjects, using magnetic resonance imaging (MRI) to assess brain volume and PET data partly gleaned from previous olfactory studies.

One possible interpretation of the connectivity pattern in straight men and lesbians is that the amygdala is wired for a greater fight-or-flight response, the authors say.