I will quote the poster "Geekgirl" of LGBT Latest Science.
Take a look at this photo and compare the brains. HeM means heterosexual man, HeW means heterosexual woman, Hom means Homosexual man and HoW means homosexual woman. If we were using these photos of brains to tell us gender, instead of our genitalia, who would you call a boy or a girl?
See the red and yellow areas? In the left amygdala, the heterosexual male brain looks very similar to the homosexual female brain. The heterosexual female brain looks very similar to the homosexual male brain. The right amygdala does not show this clear cut pattern but it is clear that there is a difference between heterosexual and homosexual men.
So where did this picture come from and what does it show?
Researchers in Sweden studied the symmetry and connectivity of the brains of straight men, straight women, gay men and lesbians. This work was published in April 2008 in the very prestigious journal Proceedings of the National Academy of Science. The original reference is:
PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects.
Authors Ivanka Savic* and Per Lindstrom
Stockholm Brain Institute, Department of Clinical Neuroscience, Karolinska Institute, 171 76 Stockholm, Sweden. PNAS April 2008.
The study itself is highly technical and not easy reading for us every day folks without this level of training. However, it can be translated into terms we can all understand. The researchers asked two main questions. Are the two halves of the brain symmetrical or asymmetrical with respect to volume? Are there differences in functional connectivity between the two halves of the brain?
The researchers chose to measure connections in the amygdala because that section of the brain has a high density of estrogen and androgen receptors. Androgens are testosterones and related hormones. A receptor allows the hormone to enter into the cells. They also chose the amygdala because of earlier research that shows this area correlates to sex and sexual orientation.
The study used ninety subjects. 25 heterosexual men, 25 heterosexual women, 20 homosexual men and 20 homosexual women. The difference in the number of subjects is not statistically significant.
The brains were analyzed using PET measurements of blood flow to measure functional connections between the two halves of the brain. In this case, a picture is able to replace a lot of words.
The researchers also looked at the volume of each half of the brain using MRI. The paper shows the variation in the data and the statistics proving the differences. For the sake of simplicity, I’ll just include the result.
HeM 25 624 612 These are significantly different.
HeW 25 581 581 These are not significantly different.
HoM 20 608 609 These are not significantly different.
HoW 20 548 543 These are significantly different.
Again, we see the same pattern. Heterosexual men and homosexual women have significantly different brain volumes from one side of the brain to the other. The heterosexual women and the homosexual men have equal brain volumes.
In other words, gay men have brains that are more like straight women than straight men. And lesbians have brains that are more like straight men than straight women.
The authors concluded that “The present study shows sex-atypical cerebral asymmetry and functional connections in homosexual subjects. The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities.”
Now, does this study prove that these differences are the cause of sexual orientation? One study alone doesn’t do that. But it is very clear that the brains of straight people and gay people are different. Not only different, but straight men and lesbians share similarities and straight women and gay men share similarities.
Let’s think about this from a different angle. If people choose to be gay, how could their brains change by making this choice? That doesn’t make any sense – and I don’t think you have to be a scientist to understand that logic.
Of course, the religiously right-winged propagandists will say, "See? God gave them over to a reprobate mind!" Those guys need to hang it up. They are on the wrong side of history!