Wired this way: sexual orientation and gender in the brain.

Born this way? Chose this way? Despite what Westboro Baptist Church & Friends might preach there is evidence (even though pretty messy) that sexual orientation and gender identity are influenced by developmental differences in our brains. Activity, connectivity and structure of certain regions have been repeatedly shown to considerably differ between gay and straight people (insert a snarky joke about bisexual erasure) as well as cis- and transgenders.
But, as with so many things in neuroscience, it is yet not 100% clear in which way the connection goes -- did these neural differences predetermine who you like or did your experiences and behaviour gradually shape these structures the way they are now? Still, a lot of scientists think these differences have been there from the very beginning, influenced by hormonal or genetic factors. So let’s see, what do we know so far about how your brain (probably) tells you who to take home!

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Prenatal development
A lot of stuff happens while you’re in the womb. You get a head, arms, legs, eyes. You start feeling pain (which, btw, doesn’t happen until around the 7th month)1, 2. You are also exposed to a lot of hormones circulating in your blood; among them androgens (such as testosterone) which are suspected to be involved in determining your sexuality. The prenatal androgen theory3 argues that differing levels of androgens in the womb influence the structure and function of brain regions which play a role in who we are attracted to. Basically it suggests that gay women have too much testosterone exposure while gay men have either too little or way too much (if only it all really was so simple).

Something you probably didn’t really expect to be seen as evidence for this theory is the finger length. Apparently, how much testosterone we got exposed to in the womb also determines how long our fingers are, so that the second-to-fourth-digit ratio is different between men and women (straight men generally have a shorter index finger as compared to the ring finger, whereas they seem to be equally long in straight women). Different studies have investigated that and it seems like lesbians tend to have a more masculine length pattern (shorter index finger), whereas gay men tend to have either hyper-masculinized or feminized ratios (much longer ring finger or equally long fingers)4. These findings are taken to reinforce the idea that an atypical testosterone exposure before birth have a say in determining your sexual orientation (but, of course, this theory is not set in stone - see there5).

Men usually have lower 2D:4D ratio than women. A popular, albeit not 100% reliable, marker for prenatal androgen exposure.

Men usually have lower 2D:4D ratio than women. A popular, albeit not 100% reliable, marker for prenatal androgen exposure.

Now, sexual preference is not the only thing prenatal hormones influence. Although it is still much disputed, gender identity also seems to be related to the androgens acting on the fetal brain. Genitals and brain do not become sex-specific at the same time -- anatomical differentiation happens in the first three months of pregnancy, while the brain follows a bit later6. Due to the different time-tables it is possible that these two processes take different routes: you might have a vagina but because of a later -- atypical for females -- testosterone surge your brain is imprinted in a male way (we’ll talk more about specific differences in the later sections).

So far the findings are quite inconclusive and messy with only one thing being clear: getting too little or too much androgens before birth most probably influences your sexual preference and/or your gender identity by making certain things in your brain different. So...what exactly is different then?

Differences in the brain
Let’s assume that hormonal/genetic factors played their role and set the brain on a non-straight development course. Now what?
Indeed, studies have found that many regions in gay brains, which normally differ between women and men, look not typical for their sex. It was found that specific regions of hypothalamus -- a region responsible for, among others, reproductive behavior and sexual response -- differ between gay and straight people.
First off, a region called INAH 3 was found to be almost double as large for heterosexual men as compared to their gay counterpart7 (since its publicaion in '91 there was only one moderately successful effort to replicate it though 8). Secondly, hypothalamus of gay men and straight women reacted to pheromones from men’s sweat (yummy!), while pheromones from female urine (it only gets better) induced a response in their smell-processing region only9 (wouldn’t surprise me if after all these sweat and urine the participants activated “leaving this stupid study”-region). Hypothalamus’ involvement doesn’t end at that: one study found that it doesn’t respond to fluoxetine, the common antidepressant, same way in gay men as it does in straight men10. For some reason this study didn’t get much attention although it has a big potential for clinical relevance: if there is a difference in the responsiveness of the serotonergic system (fluoxetine acts on serotonin) then maybe it should be considered when treating gay men with depression?

The bigger clump on the left is INH3 in heterosexual men, the smaller clump on the right in INH3 in homosexual men. From http://science.sciencemag.org/content/253/5023/1034.long

The bigger clump on the left is INH3 in heterosexual men, the smaller clump on the right in INH3 in homosexual men. From http://science.sciencemag.org/content/253/5023/1034.long

Another brain thing which was found to be very similar for gay men and straight women is connectivity pattern of amygdala, one of our emotional centers. For them, there was a stronger link between the amygdala and other regions involved in emotions. Lesbians, however, tended to be more like straight men, with stronger amygdala connections to sensorimotor cortex (among other things responsible for attending and reacting to stuff happening to you, like fight or flight reaction)11. While highly speculative, one might guess that this has something to do with the stereotype of gay men being more emotionally responsive than the straight ones (it’s not really clear what to make of all these clusters of findings other than cautiously speculate).

The amygdaloid connectivity in heterosexual men and women (labeled HeM and HeW) and homosexual and women (labeled HoM and HoW). One can see the similarities between straight women and gay men and straight men and gay women. From: http://www.pnas.org/content/105/27/9403.abstract

The amygdaloid connectivity in heterosexual men and women (labeled HeM and HeW) and homosexual and women (labeled HoM and HoW). One can see the similarities between straight women and gay men and straight men and gay women. From: http://www.pnas.org/content/105/27/9403.abstract

Differences were also found in the bigger picture: it was shown that the size of the brain hemispheres of gay men resemble this of heterosexual women (both brain halves are more or less the same size), whereas lesbians showed asymmetry similar to straight men (for these groups the right hemisphere is slightly larger). The number of nerves connecting two hemispheres of the brain in gay men was also resembling the number seen in straight women (= more than in straight men); it may be involved in sex differences relating to cognition and language 12. What is interesting about this study with the amygdala and interhemispheric connections is that it shows that regions not directly connected to sex and reproduction differ according to our sexual preference as well.

Brains of transgender people also show certain differences. Scientists found that a structure called central subdivision of the bed nucleus of the stria terminalis (if you want to sound smart-ass) or BSTc (if you’d rather go with a simple one), which is also located in hypothalamus and essential for masculine sexual behavior, is about twice as big and contains twice the number of neurons in men as compared to women. They also found that Male-to-Female (MtF)-transgenders have a female-sized BSTc (one FtM person was also tested and his BStc showed male characteristics)13, 14, 15. What’s interesting is that these differences were preceeding the hormonal therapy and were not influenced by sex hormones in the adulthood. So basically it seems that BSTc matches the gender a trans person feels they belong to and not the gender stated in their passport.
There is more: it was found that there is remarkably less androgen receptors in MtF brains as compared to cis men16. This difference could have led to a less effective testosterone signaling, resulting in a lacking “masculinization” of the brain. Also (to continue to bombard you with evidence), an MRI study found that the shape of corpus callosum, a huge bundle of fibers connecting our hemispheres which also differs between males and females, reflects transgender people’s gender and not biological sex (just like with the BSTc size).17

Left: BSTc of hetero- and homosexual men. Right: BSTc of women and MtF transgender. Source: https://www.ncbi.nlm.nih.gov/pubmed/7477289

Left: BSTc of hetero- and homosexual men. Right: BSTc of women and MtF transgender. Source: https://www.ncbi.nlm.nih.gov/pubmed/7477289

As you can see, sexuality and gender are immensely complex topics with many factors contributing to who we like and whether we feel comfortable in our body. And I haven’t even touched other important aspects having a hand in this: genetics18, 19, 20, epigenetics21, 22, birth order23, environmental influence24... Science still has a long way to go in unraveling the question of how exactly these things are determined. Still, brain-wise we have some guidance in this mess: seems like the fluctuations in the androgen level before your birth play a role in determining whether you brain will develop in the gay or straight direction and whether you will feel male or female. Anyway, whoever you decide to take home -- this cool girl or that cute guy (or both), have fun and don’t forget about protection!

A little foretaste: According to the poll the next poll will be on autism and empathy!