Sex and the brain

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Filed under Neurobiology

An interesting question is whether the brains of gay and straight people are the same or different. One study 1 found that they are slightly different and that the brains of gay men, in the aspects analyzed, are more similar to those of heterosexual women. This would support the idea that sexual orientation is a biological issue, something that is encoded in our brain circuits and that is probably from birth. It is another argument against those disgusting pseudo-therapies aimed at “curing” homosexuality, against considering it a sin or a perversion, against discriminating against someone who enriches our neurodiversity and makes us more interesting and better.

Photo: Jesse Orrico / Unsplash

The study was conducted at the Stockholm Brain Institute. Neurobiologist Ivanka Savic and her colleague Per Linström performed brain MRIs on 90 volunteers who were divided into four groups of equal numbers of participants and similar ages depending on whether they were male or female and whether they defined themselves as heterosexual or homosexual. The statistical analysis showed that the homosexual men and the heterosexual women showed symmetrical brains; that is, with the left and right hemisphere of almost exactly the same size. Lesbians and straight men, on the other hand, had asymmetrical brains, with the right hemisphere significantly larger than the left. MRI measurements showed that the right hemisphere of straight men was 2% larger than the left. Lesbians also showed asymmetry, although somewhat less, with the right hemisphere 1% larger than the left.

The results have been published in the Proceedings of the National Academy of Sciences and suggest that biological factors influence sexual orientation. It is possible that this is the result of exposure to sex hormones such as testosterone in the womb and that these factors may also shape brain anatomy. The study follows previous work that looked for differences in verbal and spatial skills in relation to genital sex and sexual orientation. The evidence showed that gay and straight women performed some language-related tasks better but had a poor sense of orientation and to move around they needed landmarks and places to refer to, while straight men and lesbians had better spatial orientation. The right hemisphere dominates spatial abilities, so it fits well that it is slightly larger in lesbians and straight men. A previous study by the same group found that gay men and straight women outperformed lesbians and straight men on tests designed to measure verbal fluency, a skill more closely related to the left hemisphere.

There are many points still to be clarified. We do not know if the differences in the size of the brain hemispheres are due to some particular structure that is involved in sexual orientation or vice versa, if sexual orientation affects the structure of the brain, which is an extraordinarily plastic organ. The group plans to investigate such brain symmetry or asymmetry in newborns, to see if they can predict what their sexual orientation will be when they reach puberty. These differences can take place in the prenatal period or begin after birth, when everyday experiences leave their mark on the brain structure but the majority opinion is that it is a combination of the two processes.

In another group of experiments, Savic and Lindström used a technique called positron emission tomography (PET) on a small group of volunteers to look at brain connections. They found that heterosexual women and homosexual men had similar connections around the amygdala, a region that plays a key role in emotional responses, and that they were different from lesbians and straight men.

Savic’s group also studied gender identity. It is one of the crucial aspects of each individual’s personal identity and is usually congruent with the sex assigned at birth and with one’s perception of body sex, but in some people this is not the case. Gender identity is a deep inner feeling by which a person feels male, female or other gender, regardless of the sex assigned at birth, and it is not well known how this is encoded in the nervous system. Individuals whose gender identity is congruent with their assigned sex at birth are called cisgender, and those whose identity resembles the opposite sex they were assigned at birth are called, as mentioned earlier, transgender. The neural circuits that encode gender identity are unknown, but the greater visibility of transgender people in democratic societies has increased interest in understanding the brain substrate of this variability.

The perception of one’s own body is modulated by the reciprocal interaction between one’s perception of one’s physical appearance, based on one’s observation and the reactions of others, and the bodily representation of one’s body in the brain. Brains appear to be different in transgender and cisgender individuals. This research group has previously published that both cortical thickness and structural and functional connectivity in the brain regions considered to be involved in the perception of one’s body differ between the two groups of people, regardless of gender and sexual orientation.

Savic’s group did this by comparing brain activation and connectivity in transgender individuals and in cisgender controls when doing a body identification task and having an MRI at the same time 2. Thirty people in each group had to view photos of their own bodies that had been manipulated to progressively resemble images of other bodies whose sex was congruent or opposite to the sex assigned at birth to the participants. The participants then had to assess the degree to which each image represented them.

While the cisgender controls identified with the images of themselves, the transgender individuals identified more with the images modified to approximate their gender identity, as opposed to their assigned sex at birth. After analyzing the effect of self-assessments, both groups activated similar systems of processing their own identity and they choose bodies that were closer to their gender identity than to their assigned sex at birth. In addition, transgender participants were more active in the limbic system, the system involved in emotions, when they saw ambiguous androgynous images of themselves transformed into their gender identity.

These results shed light on the brain networks involved in processing one’s vision and how they fit with gender identity and also show the greater involvement of emotional processing in transgender people. They open a path to understanding gender dysphoria, a condition in which transgender people report feeling strange about their own body and sometimes experiencing distaste for it, even at a very early age. It has been postulated that this dysphoria is a consequence of a different process of sexual differentiation. One hypothesis on the basis of gender dysphoria is that sexual differentiation of the genitals occurs separately from sexual differentiation of the brain in utero, making it possible that the body can veer in one direction and the mind in another.

The perception of one’s body is intimately connected with the perception of one’s self and one’s gender. Studying the neurobiology of the transgender in relation to the cisgender groups when they view their own and other people’s bodies can help to understand the biological substrate of gender identity and may be more advantageous than studying gender identity in each group separately.



  1. Savic I, Lindström P (2008) PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proc Natl Acad Sci U S A 105(27):9403-8. doi: 10.1073/pnas.0801566105
  2. Majid DSA, Burke SM, Manzouri A, Moody TD, Dhejne C, Feusner JD, Savic I (2019) Neural Systems for Own-body Processing Align with Gender Identity Rather Than Birth-assigned Sex. Cereb Cortex . pii: bhz282. doi: 10.1093/cercor/bhz282

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