# Sex differences in brain size

Next time someone asks you “Are men and women’s brains different?”, you can answer, without hesitation, “Yes”. Not only do they tend to be found in different types of bodies, but they are different sizes. Men’s are typically larger by something like 130 cubic centimeters.

Not only are they actually larger, but they are larger even once you take into account body size (i.e. men’s brains are bigger even when accounting for the fact that heavier and/or taller people will tend to have bigger heads and brains, and than men tend to be heavier and taller than women). And this is despite the fact that there is no difference in size of brain at birth – the sex difference in brain volume development seems to begin around age two. (Side note: no difference in brain volume between male and female cats).

But is this difference in brain volume a lot? There’s substantial variation between individuals, as well as across the individuals of each sex. What does ~130cc mean in the context of this variation? One way of thinking about it is in terms of standardised effect size, which measures the size of a difference between the two population averages in standard units based on the variation within those populations.

Here’s a good example – we all know that men are taller than women. Not all men are taller than all women, but men tend to be taller. With the effect size, we can precisely express this vague idea of ‘tend to be’. The (Cohen’s d) effect size statistic of the height difference between men and women is ~1.72.

What this means is that the distribution of heights in the two populations can be visualised like this:

With this spread of heights, the average man is taller than 95.7% of women.

Estimates of the effect size of total brain volume vary, but a reasonable value is about ~1.3, which looks like this:

This means that the average man has a larger brain, by volume, than 90% of the female population.

For reference, psychology experiments typically look at phenomena with effet sizes of the order ~0.4 , which looks like this:

And which means that the average of group A exceeds 65.5% of group B.

In this context, human sexual dimorphism in brain volume is an extremely large effect.

So when they ask “Are men and women’s brains different?”, you can unhesitatingly say, “yes”. And when they ask “And what does that mean for differences in how they think” you can say “Ah, now that’s a different issue”.

Kristoffer Magnusson’s awesome interactive effect size visualisation

Previously: gendered brain blogging

Edit 8/2/17: Andy Fugard pointed out that there are many different measures of effect size, and I only discuss/use one: the Cohen’s d effect size. I’ve edited the text to make this clearer.

Edit 2 (8/2/17): Kevin Mitchell points out this paper that claims sex differences in brain size are already apparent in neonates

## 10 thoughts on “Sex differences in brain size”

1. Such a good explanation, thank you, will be using it… a lot

1. Deirdre Di Donna says:

Then again, women have a larger corpus callosum, connecting the left and right hemispheres, probably improving the integrated usage of the capacities of each. The brain is so complex. Let’s not stop at “who’s is bigger,” which is such a male obsession, anyway…

1. Given no judgement was made on quality vs quantity, and certain groups claim that there is *no* difference between male & female, this difference in size is a useful data point in refuting said claims.

Women are significantly better at verbal communication. I don’t remember seeing men recommending we ignore such superior differences due to potential for perceived inferiority.

2. Paolo says:

I’m only a layman when it comes to this stuff, but I read a paper by Arthur Toga and Paul Thompson that claims brain size and IQ are strongly correlated. Why is it that Asians and Jews who have lower global average heights tend to have higher global average IQs? Do they adjust their concept of brain size by the height – a “real size”? Again, not experienced in this domain.

1. tomstafford says:

Here is the paper you refer to

Toga & Thompson, 2005, which is one of a number asserting that brain size and IQ *do* correlate (also Rushton & Ankney, 1995, Taki et al, 2012 ). I’m afraid I don’ know the answer to your question. I think the height -brain size correlation is weak adults, which might go some way to answering it

In other news: No sex differences in language proficiency: Wallentin (2009)

Sex differences in brain volume are related to specific skills, not to general intelligence

1. Paolo says: