I remember reading an article a long time ago by a female brain scientist who said nothing made her crazier in our modern discourse than the common assumption that male and female brains are physically the same.

Scientists used to believe that both gender’s brains identical, but closer study in rats and humans in the 1970s and 80s revealed that some parts of the brain are have different structures for different genders. Early research then looked for brain differences between gay and straight people.

That big news came from an announcement by Dr. Simon LeVay in 1991. Examining the hypothalamus from the brains of gay men who died of AIDS, and comparing them to the brains of deceased straight men, he found that gay men’s hypothalmus’ were half the size of straight men’s, a proportion more like that of women. Although this early study was subsequently questioned, Dr. LeVay’s work opened up the study of the physiology and genetics of gay people.

One of the gendered differences is in brain symmetry. Straight women’s brains are symmetrical, with both hemispheres of equal size, while straight men are asymmetrical, with a larger right hemisphere. Swedish researchers used MRIs to compare brain hemispheres of straight and gay men and women. They found that the physical structures of gay men’s brains were more like heterosexual women’s, and lesbians brains were more like those of straight men.

Beyond brain symmetry, these researchers did positron emission tomography (PET) scans to measure blood flow to the amygdala of each hemisphere. Straight men and gay women had more nerve connections in the right side of the amygdala, while straight women gay men had more neural connections in the left amygdala. The amygdala is the center of emotional learning and memory consolidation, affecting behavior as part of our “flight, flight, or mate” response (although it is easier to remembered with three Fs, ahem). As a British scientist told the BBC:

In other words, the brain network which determines what sexual orientation actually “orients” towards is similar between gay men and straight women, and between gay women and straight men.

This Swedish study stands out for another reason. Because money for gay research is usually blocked by social conservatives, those who do gay research are often gay themselves, opening the results to charges of bias. This Swedish study was not looking for any findings on gay people. They were studying strokes, asked participants if they were gay or straight as part of the intake process, and found these results in the subsequent data.


  • Gay Men: Tend towards symmetrical brains and increased amygdala neural connections on the left side resembling straight female brain structures, indicating basic other-gender similarities from birth.
  • Lesbians: Tend towards asymmetrical brains and increased amygdala neural connections on the right side resembling straight male brain structures, indicating basic other-gender similarities from birth.

  • Simon LeVay, PhD, et al., A Difference in Hypothalamic Structure Between Heterosexual and Homosexual Men, Science, June, 1991
  • Ivanka Savic-Berglund, MD, PhD, Proceedings of the National Academy of Sciences, June 16, 2008

This article is part of a series, Written on the Body, exploring the correlations between our body structures and sexual attraction.