Darwin insisted that his theory explained not just the complexity of an animal's body, but the complexity of its mind. "Psychology will be based on a new foundation," he famously predicted at the end of The Origin of Species. But Darwin's prophecy has not yet been fulfilled. More than a century after he wrote those words, the study of the mind is still mostly Darwin-free, often defiantly so. Evolution is said to be irrelevant, sinful, or fit only for speculation over a beer at the end of the day.

Pinker's goal throughout How the Mind Works is to convince us that, in fact, an evolutionary approach is central to "reverse engineering" the mind. In this he largely succeeds, and, as is so often the case in accounts of evolution, he does so through sheer weight of evidence. There are innumerable aspects of the mind that can be quite reasonably explained by natural selection, and Pinker goes through them all until even a skeptic would have trouble dismissing them.

For nonskeptics, of course, who are already predisposed to think of the brain as just another bodily organ, Pinker's explanations are even easier to swallow. Natural selection, once the light bulb goes on and you understand it properly, is so immensely powerful as an explanatory theory that it's hard to ever again consider any aspect of any form of life--body or brain--without being easily convinced that it's a product of evolution.¹

Still, it's the specific explanations for the various attributes of mind that are most interesting, and this raises the biggest problem in Pinker's book, namely that it's pretty easy to come up with multiple explanations for any adaptation you can think of, all of which are plausible from an evolutionary point of view but none of which can really be proven. In the end, Pinker offers plenty of explanations of his own, and they are mostly pretty convincing, but, really, who knows? Even if the basic evolutionary structure of the mind remains unchallenged, it's quite possible that every single specific explanation in How the Mind Works could turn out to be wrong. Or not.

So with that caveat out of the way, what does Pinker have to say? Here's a sampling:

• Pinker's primary goal is to explain human thought via the "computational theory of mind," a fairly recent recent model that treats the brain as a collection of specialized modules, each of which does something that at one time or another was evolutionarily useful. I'm fond of this model myself, but Pinker goes on to deflate my preconceptions by warning that "the claim is not that the brain is like commercially available computers":

To explain how birds fly, we invoke principles of lift and drag and fluid mechanics that also explain how airplanes fly. This does not commit us to an Airplane Metaphor for birds, complete with jet engines and complimentary beverage service.

However, despite this warning, Pinker goes pretty far down the road of treating the brain an awful lot like the commercially available computers he cautions against, right down to "logical and statistical operations directed by comparisons, tests, branches, loops, and subroutines embedded within subroutines":

Human thought and behavior, no matter how subtle and flexible, could be the product of a very complicated program, and that program may have been our endowment from natural selection. The typical imperative from biology is not "Thou shalt...," but "If...then...else."

• If the mind has been shaped by the iron hand of evolution, does that mean that environment plays little part in shaping our personalities and our culture? Pinker gives one of the best answers I've seen to this question:

If the mind has a complex innate structure, that does not mean that learning is unimportant....It's not that the claim that there is an interaction between innate structure and learning (or between heredity and environment, nature and nurture, biology and culture) is literally wrong. Rather, it falls into the category of ideas that are so bad they are not even wrong.

As Pinker points out, having lots of complex, innate structure makes us respond more acutely to input from our environment, not less.

• Why is the mind so overdeveloped? The evidence seems to indicate that humans have way more intelligence than they could possibly have made use of 100,000 years ago, so what adaptive purpose did it serve? Pinker offers up a recent theory that claims it was due to a sort of arms race between cheaters and people trying to detect cheating. Just as peacocks' tails became bigger and bigger due to competition between males, even to the point where they were positively detrimental to the peacocks' health, the human mind developed because this made it more effective in finding ways to deceive fellow humans in social situations. This in turn caused humans to become ever more sensitive to deceit, and so forth. It is only by chance, Pinker says, that the same faculties that allow us to deceive and to detect deceit are also pretty useful for performing calculus and creating Web pages.

• Why do we have violent emotions? In what scenarios is it actually better to lose control of yourself rather than acting coolly and rationally? Once again, the answer has to do with social interaction. Humans are forever negotiating with each other, and the core problem of negotiation is convincing your adversary that you have made your best offer and are unwilling to back down any further. One way of doing this, Pinker suggests, is to become furious: when you visibly lose control your adversary knows that he's gotten as good a deal as he's going to get and it's time to compromise. This also helps explain why emotions are mirrored so directly in our facial expressions: it's necessary to advertise your emotions in order to convince your opponents that you've really lost control and aren't just faking it. (Of course, over time humans have, in fact, gotten better at faking it, and also gotten better at detecting fakery. This is part of the arms race described above. However, the quality of fakery is inherently limited because the facial expressions of genuine emotions are controlled by a different part of the brain than fake emotions, and it is extremely difficult to learn the muscle control necessary to do a first class job of faking emotion.)

• Why do autostereograms work (you know, those random dot patterns that turn into 3-D pictures if you stare at them long enough)? Pinker uses this as the springboard to a lengthy chapter on how our visual system evolved, which does a good job of describing just how sophisticated and flexible our eyes really are. Here's the nickel version of how autostereograms work:

1. First, consider that humans have two eyes, each of which sees a slightly different view of the world. How does the brain decide that a particular speck in one eye and a particular speck in the other eye are actually the same object? Two things are at work:
2. First, the brain has some highly specialized centers that process visual data and compare the input from both eyes. If a similar feature is seen in almost the same place in both eyes, the processing center merges the two views, interprets the feature as a single object, and creates a 3-D view of it in our mind.
3. Second, if you look at something close up, your eyes converge. If you look at something far away, they diverge. If you look at something very far away, they diverge until they are nearly parallel. The brain decides whether things are in "almost the same place"--and merges the views coming from each eye--based on the degree of eye convergence.
4. Autostereograms are created by drawing two identical pictures out of dots and placing them about half an inch apart, surrounded by a bunch of random noise. In order for the brain to interpret this as a single 3-D picture, it's necessary for the eyes to look straight ahead (i.e., not converge on the page), fooling the brain into thinking that the two similar looking patterns it sees are actually a single object being viewed through two eyes.
5. The problem with this is that the focusing muscles of the eye are directly hooked up to the muscles that control eye convergence. Thus, if you stare into the distance, as you need to do in order to resolve the picture, they will automatically focus to infinity.
6. Autostereograms work only if you can break the connection between convergence and focus. When you stare at an autostereogram, your brain is feverishly trying to make sense out of the image and eventually it begins to realize that the separated dots might actually be a single object. However, because your eyes are converged on infinity, they are also focused to infinity. The illusion snaps into place when your brain starts to see the pattern and you can refocus your eyes on the page without changing your convergence.

Some people (about 2% of the population) are unable to do this at all, while others find it fairly easy. It's also possible, according to Pinker, to train yourself to control focus and convergence separately if you're willing to spend a few hours at it.

One of the points Pinker illustrates with this explanation (which goes on for about 30 pages) is the difficulty that the brain has in making sense of the world, and how many built-in assumptions it uses to do so. For example, if the eye sees an oval, is it really an oval seen head on, or is it a circle seen at an angle? Both are equally possible, and the brain must make some assumptions in order to come up with its best guess of what reality really is. Among other things, "the human visual system 'assumes' that matter is cohesive, surfaces are uniformly colored, and objects don't go out of their way to line up in confusing arrangements." These assumptions exist in the brain thanks to natural selection: brains that made them were better able to make sense of the world than brains that didn't, because 100,000 years ago these assumptions were nearly always accurate. Pinker explains, "That is why psychologists are obsessed with illusions. They unmask the assumptions that natural selection installed to allow us to solve unsolvable problems and know, much of the time, what is out there."

• Why do we enjoy music? Pinker doesn't know, and he uses up several pages talking about our love of music precisely because it is one of the things that is still unexplained. Apparently nobody has yet come up with any good explanation of why an appreciation of melody and rhythm should have been adaptive 100,000 years ago, despite the fact that virtually every culture on earth has some sort of musical appreciation. Still, his discussion is interesting, and marked the first time that I've read an explanation of the chromatic scale and the concept of "key" that I sort of understand.

Pinker is a good writer who knows how turn a phrase (he calls the ascendancy of homo sapiens "the ultimate revenge of the nerds" and says about natural foods, "Your local Happy Carrot Health Food Store notwithstanding, there is nothing particularly healthy about natural foods."). What's more, I noticed that whenever he made a stray comment about something outside his field that I happen to know something about, he was always accurate both factually and philosophically. This is unusual, and provides some confidence that he is a careful researcher both within his field and outside of it.

It's hard to do justice to a book like this because it truly gets its power from its scope rather than from a few specific points it makes. Explaining particular bodily functions as adaptations to particular conditions in the past is probably a mug's game at best, and more than a few such explanations have looked silly when new evidence was uncovered. However, taken as a whole the theory of evolution via natural selection is intensely compelling (not to mention beautiful and awe inspiring), and this is equally true whether you're talking about hearts and livers or amygdalas and prefontal cortexes.

Pinker is, unfortunately, forced to spend a fair amount of time clarifying, over and over, that simply because some particular behavior is a result of evolution does not mean that it is inevitable and unalterable, nor does it mean that it is either moral or beneficial. (At least I assume that he was forced into it, most likely from long experience of being assaulted unless this clarification is made again and again). This has always struck me as an odd conclusion in any case: after all, shouldn't knowledge of an innate predisposition make us more likely to fight it than to simply give in to it? The fact that some set of behaviors was adaptive 100,000 years ago on the African savannah certainly doesn't mean it's adaptive (or moral or beneficial) in today's artificial world, and you'd think people could understand this concept.

How the Mind Works is a good book and an eye opening one. Given the current state of knowledge, it's inevitable that some of Pinker's statements are speculative (his discussion of consciousness, for example, is sketchy and unhelpful) but overall it's a thorough and readable explanation of how natural selection has influenced who we are and how we behave. Recommended.

^[1] Note that "properly" is the operative word when it comes to understanding evolution, and I was forcefully reminded of this when I started reading a volume of world history right after I finished How the Mind Works. In a mind bending misunderstanding of how natural selection works, the author states, "Even the skeptical must accept that no system so intricate as that of life has occurred by chance." No kidding. Of course, maybe I shouldn't take this too seriously since a couple of sentences later he also says, "Even the agnostic scientist Laplace...agreed with Newton that the way that the planets revolved in the same direction round the Sun was unlikely to be a matter of chance." Sure, this guy is a history professor, but even so it's always freshly disappointing to run into someone so aggressively and willfully ignorant of even the basics of modern science.

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