Brain Size Redux

My Dad told me a story once. He was taking calculus and the professor had put up a particularly nasty equation. The professor looked at it for a while and then said he ought to be able to derive that. Forty sweaty minutes later he had done so, washed the chalk from his hands and left the room.

I remembered this when It was pointed out to me in a recent political discussion that one should put up or shut up. I got to thinking about my last post whining about some bad science in the context of human narcissism. The example was an unfortunate paper that made some poor conclusions on SAT scores based on the difference between brain size between men and women.

Well, could I do better? In the memory of that long dead professor I resolve to try.

Always go to first principles. In this case what is bigger between men and women?

A quick look at the literature gave me this little gem of a paper by Eileen Luders. (See here.)

The brain size issue is actually a bit more complex than at first glance. Yes, men have, on average, brains about 13-15% larger than women. Women, for that matter, have more gray matter volumes than men. Gray matter is composed of cell bodies. White matter, the other component, is composed mostly of axons and glial cells. If this were a telephone system, the gray matter would be the switchboards and the white matter the wires. And if you get this metaphor then you're as old as I am.

Luders and her colleagues took an interesting approach. They compared a collection of brain images of men and women that were the same size-- it turns out that there is an overlap where men of small brains and women of large brains have comparable sizes. They also compared the smallest brains of women to the largest brains of men.

Their results were interesting. First, there were differences in total brain volume between men and women across the total sample. But when the comparison was made between men and women who had brains of the similar size no significant differences in total brain volume. However, there were differences in specific gray matter ratios. For example:

"...our findings of increased GM concentration in the left posterior superior temporal gyrus. Given that parts of the superior temporal gyrus are involved in language processing, one could speculate that the observed larger GM volumes in females are associated with women's superior language skills."

To be entirely fair there were no indications mentioned in the article where regional gray matter ratios showed greater for men then women. That's pretty one sided and suggests there might be more to the story. In addition, these were studies where only brain size, volume and regional distribution were compared. There was no indication of body size in the data which is where the original size differences were noted. If the brain volumes of a man and a woman are compared as the same size and the man is five feet tall and the woman is five foot nine, that's significant data. Remember, the original data was a proportional comparison of brain size as related to body size. Further there is no comparable data referenced in the paper to living non-human primate species.

Let's presume Luders' paper is correct-- I have no reason to doubt it. Let's also presume Schoenemann's work, the source of the data in my previous post, is also correct. This gives us a state where brains of equivalent size are of equivalent volume regardless of sex. It also gives us a brain mass difference that is beyond what is expected from body mass difference.Again, we first have to think about the data.

The first question involves the methods of the two works. One study compared brain volume and the other compared weight. Could the sex differences disappear if equivalent measurement techniques were used?

Possibly. I suspect it unlikely. That would suggest to me a more fundamental difference in brain structure between men and women and Luders' paper suggest that though there are significant differences in brain gray matter between women and men they were not on the order of 3% of total mass. That's a lot.of grams.

We can also look at the difference in samples between the two studies. Luders' paper used brain scans of living men and women. Scheoneman's work used post mortem brains. Could there be a difference between the way men and women's brains reacted to preservation.

This is a really good question and I don't have a good answer for it. Tissues do react differently to preservation. However, Luders' work again didn't show a lot of structural differences between men and women and I think we could presume that gross biochemical differences, at least on the order of tissue differences, between the brains of men and women would show up before now.  So, again, I'll say unlikely.

That leaves us with a significant difference in brain sizes between men and women, relative to body size.Why? Brain tissue is extremely expensive. We can't use Luders' paper for this one: she didn't find anything that was significantly larger in men's brains compared to women's. We're back to speculation.

Well, I'm a science fiction writer. Speculation is what I do best.

I keep going back to the chimp data. We can make all sorts of cognition comparisons between men and women but that number (human 3%, chimp 4%) is astonishingly similar. It suggests that whatever the reasons for brain size difference between men and women they're likely the same for chimps. It's not language processing or SAT scores. It's something much more fundamental.

Well, one idea that leaps to mind is that we're wrong in how we're evaluating brain size/body size relationships. We're presuming a linear proportion. I.e., if the body is 10% bigger we're expecting a 10% bigger brain. It's possible that the proportion (at least in the great apes) is non-linear. Instead of y=mx it's y=x**m. This could probably be modeled across primates and determined. But that is not this day. Besides, I'm and SF writer. A simple mistake in proportion would never get me a story.

While I was working on this post another idea suggested itself to me. Let's think about sex at a very basic level. You have two animals that are looking out for their own survival. It may not be in the interest of their personal survival to have a sexual encounter. However, it is in the interest of their species survival that the sexual encounter occur. Think spiders. A male spider that is coming to mate with a female has to signal to her that he is not prey otherwise he might get eaten. There is a tension between individual needs and species needs.

Think of chimps and humans-- unfortunately, I have to neglect bonobos because I don't have any data. The male of both species needs aggression to achieve dominance. The male must also turn off that aggression be able to cooperate with other males and be able to get a mating opportunity. There is aggression in chimps towards females but you don't get alpha status unless you can get the females to accept you and that requires a greater repertoire than just beating them over the head. Could the extra brain tissue in males (both chimp and human) be required to inhibit aggression?

There could be some fruitful research here. Do the bonobos have a similar proportional difference? Baboons are aggressive when they're trying to enter a new band. They're trying to get noticed by the females. As soon as they are they change behavior to gentle supplication. (See Shirley Strum's book, Almost Human, which I'm hopefully not misremembering.) Is there a brain difference there?

Regardless, as I said last time there are brain differences between men and women but they are long standing and don't have a lot to do with modern life. Luders' work says, to me, that they are greatly outweighed by the similarities.

I hope I derived the hell out of that equation.