In defense of evolutionary psychology, part 2: adaptationism and the reason human brains are so big

So in the first post of this series, I quoted the editors of The Adapted Mind as saying in effect that we can assume complex features of the human mind are evolutionary adaptations. And I know this is going to get them accused by some people of “adaptationism,” which is the alleged vice of assuming things are evolutionary adaptations despite of not having any good evidence that this assumption.

Sometimes these criticisms take the form of saying “in order to establish that something is an adaptation you have to do X, and you haven’t done X therefore your claim isn’t good science.” And I’m going to be frank, I think some people set the bar way too high for showing that something is an adaptation, and if your theory of science says that we can’t know whether or not certain things are adaptations so much the worse for your theory of science. In that case it’s your theory of science that’s flawed, not evolutionary psychology.

I think Steven Pinker nailed this one in How the Mind Works:

One claim is that reverse-engineering, the attempt to discover the functions of organs (which I am arguing should be done to the human mind), is a symptom of a disease called “adaptationism.” Apparently if you believe that any aspect of an organism has a function, you absolutely must believe that every aspect has a function, that monkeys are brown to hide amongst the coconuts. The geneticist Richard Lewontin, for example, has defined adaptationism as “that approach to evolutionary studies which assumes without further proof that all aspects of the morphology, physiology and behavior of organisms are adaptive optimal solutions to problems.” Needless to say, there is no such madman. A sane person can believe that a complex organ is an adaptation, that is, a product of natural selection, while also believing that features of an organism that are not complex organs are a product of drift or a by-product of some other adaptation. Everyone acknowledges that the redness of blood was not selected for itself but is a by-product of selection for a molecule that carries oxygen, which just happens to be red. That does not imply that the ability of the eye to see could easily be a by-product of selection for something else (pp. 165-166).

To give an example of this, let’s talk about human brain size. Human brains are very large relative to their bodies. Some animals like whales may have larger brains, but if you graph animal brain size against body size for many different animals, humans are very clearly an outlier. Why is this? Could it be a byproduct of something else rather than an adaptation?

Well, no. The problem is that evolutionarily speaking human brains are really expensive. First of all, it’s hard to get a head that big through the birth canal, even when you’re talking about a baby’s head which is smaller than an adult human’s head. Those big heads we have are the reason that women dying in childbirth used to be such a big problem before the coming of modern medicine. Also, brains are expensive in terms of metabolic energy. They’re a tiny fraction of our total body mass, but they use something like 20% of our daily energy intake.

If human brains were just an unfortunate side effect of something else going on in human evolution, two things would have to be true. First of all, whatever was driving that other thing would have to be so powerful as to be able to overcome the strong selection against big brains for reasons of death in childbirth and the energy cost.

Second, it would have to be true that spite of the big selection pressure, evolution would have to turn out to be unable to decouple brain size from whatever that other thing was. And that seems extraordinarily improbable. That gives us pretty good reason to think that there must be a big evolutionary benefit to having large brains, large enough to outweigh the costs. Even if we don’t know what that benefit is.

The situation with language is similar, though not because being able to speak a language is so costly, but just because the ability to speak a language is clearly a complex feature of the human mind. Linguists have realized since about the time of Noam Chomsky in the 50s that if we learned language through a totally general-purpose learning mechanism, we would never learn languages fast as we do.

If you look at how toddlers learn language, they don’t learn it through simple induction. The only real way to explain why we would have these language learning mechanisms is that the ability to learn language gave us some kind of benefit in our evolutionary past. (I won’t go any further into the details of language here, but if you’re interested in that subject, I recommend another book by Stephen Pinker, The Language Instinct.)

Now I don’t claim to know for sure what exactly was the main driving force in the evolution of language and the evolution of human intelligence more generally. Was it that these things made us better hunter-gatherers? Did they evolve to help us better plot against rivals? Did they evolve to help us impress members of the opposite sex, as Geoffrey Miller proposes in his book The Mating Mind?

I’m not sure. I suspect it was a combination of all these things, but I don’t really know. But that doesn’t stop me from being very confident that they are adaptations for something, and we shouldn’t be so cautious about seeing traits as adaptations as to stop ourselves from saying that. And the difficulties here shouldn’t stop scientists from at least trying to make educated guesses about what drove the evolution of human intelligence.

Note: edited for paragraph breaks.

Angry Atheists? chapter 8: Science and Religion II: Evolution
Science!
Bill O'Reilly's argument for the existence of God
Peter van Inwagen's argument for Christianity

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