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I think we've all heard this saying: "Men are from Mars, women are from Venus." The implication being that they are just completely, totally, fundamentally distinct. But say you picked up a male brain and a female brain and compared them.

Like, you squeezed them and you poke 'em and prod 'em and... This is kind of a gross example. I don't think we should be playing with brains.

Put the brains down! The question is: if you looked at a male and female brain side by side, would you be able to see any differences? Well, the answer is super controversial, and you probably wouldn't be able to tell with the naked eye.

But, like... maybe? The better question is, would it even matter if you could? Now, it's important to note that we're talking about biological sex here, not gender.

Almost all of the studies on brain anatomy have been done with people whose gender identity aligns with the sex they were assigned with at birth. Which means we can't really apply their results to people with other gender identities. Same for people who are intersex, meaning their chromosomes or reproductive anatomy don't fit the typical binary.

With that in mind, though, we can ask whether there are. Fundamental differences in brain anatomy between biosex males and females. And there are a lot of good reasons to ask that.

Outdated stereotypes, like the idea that males are better at math, aren't one of them. But there are differences between males and females that are pretty well supported. For instance, females are more frequently diagnosed with disorders like depression and anxiety, and have higher rates of Alzheimer's.

Biological differences in the brain, if they exist, could help explain and treat these diseases. And then there's the fact that most of science just doesn't use female animal models. A 2010 study of sex bias and research found that neuroscience was one of the guiltiest fields.

And that male animals were studied more than five times as often as female animals. There are reasons for this, including the fact that lab mice go into heat every four to six days. I mean if you thought PMS once a month was bad try factoring that kind of hormonal haywire into your nicely controlled scientific experiment.

But if human male and female brains are different then not studying female animal models is a pretty big problem. So are sex related anatomical differences a thing or not? Well, yes and no.

The one consistent finding is that male brains are bigger on average than female ones would make sense, because generally male heads are bigger. Bigger bodies, bigger body parts. Other studies suggest that there are differences in specific brain regions Or that for females some regions have thicker cortices: the folded gray matter on the outer edge of the brain that we use to do our higher-level cognition, but findings vary by study and all of them depend on whether or not you correct for the larger size of the male brain overall.

When you do, some of the results disappear, and some stay the same, and some even flip which sex is larger. Though, again, different studies have had different results and whether or not to do that kind of size correction is still hotly debated by scientists. Some researchers point out that larger brain structures are still larger brain structures no matter what size your body is.

Those extra neurons got to be doing something, right? And we make all this fuss about our brains being bigger than those of other primates, because we assume that size matters. But it's not like we think sperm whales are vastly more intelligent than we are, even though their brains are more than six times the size of ours.

So most studies investigating behaviors or disease conditions look at the relative size instead. Indeed the very fact that a lot of sex differences disappear once you correct for relative head size suggests that maybe it's worth taking a second look at. For example, a 2008 teen study, which is perhaps the most comprehensive to date looked at the brains of 2,750 females and 2466 males, and found that anatomical differences did exist and were statistically significant.

On average females had thicker cortices in 48 of the 68 brain regions examined, but males had larger brains and larger brain regions and structures. When size of the brain was accounted for, though, the sex differences and the size of most of these regions disappeared. So do the size differences in structures like the hippocampus, caudate nucleus, and thalamus.

And females ended up having larger relative volumes for 10 regions as well as the right nucleus accumbens. But the differences in size of these structures absolute or relative were small and there was a lot of overlap. So if a particular structure was really big were really small it might be associated with a particular sex.

But plenty of different sizes for a given structure could be considered totally normal for males or females. And previous studies had found that while there are average differences between the sexes, individuals don't necessarily have male or female brains. Researchers in a 2015 study of 281 brains found that individuals often have like a "Mosaic of male-ish and female-ish brain characteristics rather than all their structures tending toward one sex." What conclusion can be drawn from all of this?

Well, despite all the controversy there might be some small differences on average between male and female brains. And that does kind of make sense. After all males and females have different chromosomes and are exposed to a different set of hormones both prenatally, and throughout our entire lives.

Despite that, though, a lot of social psychologists and sociologists point out that it's impossible to know how much of these sex differences come from biology and how much come from environmental influences. The brain is famous for being plastic after all. And it makes sense that if playing video games alters your brain, how you were raised and treated and act alters your brain too, but there's also an assumption underlying all this debate which is rarely questioned that these differences correlate to sex specific differences in cognition or behavior.

The thing is there's not much evidence that that's true. And there might be a reason for that. One interesting theory some psychologists have is that size does matter.

But that the anatomical differences between the sexes compensate for any cognitive differences that might otherwise arise because of sizing. So that relatively large right nucleus accumbens or the thicker cortices of females don't make them act or think differently than males. It ensures they don't, despite the male's larger noggins.

And even if size does matter, bigger isn't necessarily better. There simply isn't a lot of evidence that actually connects anatomical differences in brains to behavior or cognition unless you're talking about things like malformations, lesions, or serious brain damage. And remember phrenology?

That super cool trend in the 1800's of using the lumps and bumps on a person's head to make judgments about their thoughts and personality? Yeah, that one made everybody look silly. So as usual there's a lot more work to be done to figure out what all this means.

But for now you can be pretty skeptical of anyone trying to use brain anatomy to justify any argument about differences between the sexes. Thanks for watching this episode of Scishow Psych. If you want keep exploring the wonders of brains large and small, be sure to click that Subscribe button. *Theme Plays*.