LONDON Gay men and straight women share some characteristics in the area of the brain responsible for emotion, mood and anxiety, researchers said on Monday in a study highlighting the potential biological underpinning of sexuality.
Brain scans also showed the same symmetry among lesbians and straight men, the researchers wrote in the Proceedings of the National Academy of Sciences.
"The observations cannot be easily attributed to perception or behavior," the researchers from Sweden's Karolinska Institute wrote. "Whether they may relate to processes laid down during the fetal or postnatal development is an open question."
A number of studies have looked at the roles genetic, biological and environmental factors play in sexual orientation but little evidence exists that any plays an all-important role. Many scientists believe both nature and nurture play a part.
Brain scans of 90 volunteers showed that the brains of heterosexual men and homosexual women were slightly asymmetric with the right hemisphere slightly larger than the left, Ivanka Savic and Pers Lindstrom wrote. The brains of gay men and heterosexual women were not.
Then they measured blood flow to the amygdala -- the area key for the "fight-or-flight" response -- and found it was wired in a similar fashion in gay men and heterosexual women as well as lesbians and heterosexual men.
The researchers added that the study cannot say whether the differences in brain shape are inherited or due to exposure to hormones such as testosterone in the womb and if they are responsible for sexual orientation.
But this is something they plan to look at in a further study of newborn babies to see if it can help predict future sexual orientation.
"These observations motivate more extensive investigations of larger study groups and prompt for a better understanding of the neurobiology of homosexuality," they wrote.
(Reporting by Michael Kahn; Editing by Will Dunham and Ralph Boulton)