June 16 (Bloomberg) -- Gay men and straight women share brain characteristics that suggest sexual preferences may be innate rather than learned, researchers said. Lesbians and heterosexual men also had similar brain tendencies.

A study of 90 adults showed similarities between gay men and straight women in a part of the brain linked to emotional response called the amygdala, and a similar finding for lesbians and straight men. The research also found lesbians and heterosexual men had larger right brains, the side associated with spatial ability, while the left and right brains of both gay men and straight women were more symmetrical.

The study, published in the Proceedings of the National Academy of Sciences, adds to research that suggests a biological basis for homosexuality, researchers said. Earlier studies have mostly focused on behavioral differences and similarities.

``It's likely that these differences are not influenced by learning or socialization,'' said Qazi Rahman, a lecturer in cognitive biology at Queen Mary University of London. ``We've known for some time that homosexuals of both sexes may show differences in certain abilities, which are known to reside on one or the other of two sides of the brain.''

Rahman was not involved with the research, which was done by Ivanka Savic and Per Lindstrom, of the Stockholm Brain Institute. The reason for the similarities between the brains of straight people and gay people of the opposite sex is unknown, the authors wrote.

Other Reasons

While the results were striking, they would be more convincing if the authors had matched the groups for IQ, education and measures of depression and anxiety, said Suzanne Corkin, a professor of behavioral neuroscience at the Massachusetts Institute of Technology, in an e-mailed statement. Also, the authors are ``overly dismissive'' of the potential role of environmental influences, Corkin said.

``In short, I would be reluctant to draw strong conclusions about heterosexual versus homosexual brain structure and connectivity from this single experiment,'' Corkin said. She wasn't involved in the study.

Gay men, like heterosexual women, outperform straight men on verbal tests. Heterosexual men do better than gay peers on navigation and on tests identifying shapes, according to background contained in the paper. There isn't much data on lesbians, the authors wrote.

Genetic factors are unlikely to have caused the differences, the authors wrote. Exposure to testosterone before birth has been shown to alter development in animals and may account for the difference, the article said. The authors said it wasn't clear whether the animal data apply to humans.

The study wasn't designed to evaluate whether changes took place before or after birth. The differences can't be ``easily attributed to perception or behavior,'' the authors wrote.

The study assessed brain volume using MRI scans, and PET scans from previous studies to show the connections in the amygdala.

To contact the reporter on this story: Elizabeth Lopatto in New York at elopatto@bloomberg.net.

Last Updated: June 16, 2008 15:00 EDT