Human behavior is notoriously complex, and there's been no shortage of psychologists and psychological theories venturing to explain what makes us tick. Why do we get irrationally jealous? Or have midlife crises? Why do we overeat to our own detriment? Why do we find ourselves often strongly attracted to certain physical traits? Numerous theories abound, but few are perhaps as novel and thought-provoking as those suggested by a new book with a long title: Why Beautiful People Have More Daughters: From Dating, Shopping, and Praying to Going to War and Becoming a Billionaire -- Two Evolutionary Psychologists Explain Why We Do What We Do. Written by Satoshi Kanazawa and Alan S. Miller, the book finds answers not in ids, egos and superegos, but in the evolution of the human brain. Written in snappy prose, their argument is essentially that our behavior — our wants, desires and impulses — are overwhelmingly shaped by the way our brain evolved 10,000+ years ago, and one consequence is that our ancestral brain is often responding to a world long ago disappeared, not the modern, fast-changing world in which we live. This disconnect can lead us to be out of sync, to act in ways that seem inexplicable or counter-productive, even to ourselves. These arguments belong to new field called "evolutionary psychology," and we were fortunate to interview Satoshi Kanazawa (London School of Economics) and delve further into evolutionary psychology and the (sometimes dispiriting) issues it raises. Have a read, check out the book, and also see the related piece that the Freakonomics folks recently did on this book. Please note that the full interview continues after the jump.

DC: In a nutshell, what is “evolutionary psychology”? (e.g. when did the field emerge? what are the basic tenets/principles of this school of thinking?)

SK: Evolutionary psychology is the application of evolutionary biology to human cognition and behavior. For more than a century, zoologists have successfully used the unifying principles of evolution to explain the body and behavior of all animal species in nature, except for humans. Scientists held a special place for humans and made an exception for them.

In 1992, a group of psychologists and anthropologists simply asked, “Why not? Why can’t we use the principles of evolution to explain human behavior as well?” And the new science of evolutionary psychology was born. It is premised on two grand generalizations. First, all the laws of evolution by natural and sexual selection hold for humans as much as they do for all species in nature. Second, the contents of the human brain have been shaped by the forces of evolution just as much as every other part of human body. In other words, humans are animals, and as such they have been shaped by evolutionary forces just as other animals have been.

DC: Evolutionary psychology portrays us as having impulses that took form long ago, in a very pre-modern context (say, 10,000 years ago), and now these impulses are sometimes rather ill-adapted to our contemporary world. For example, in a food-scarce environment, we became programmed to eat whenever we can; now, with food abounding in many parts of the world, this impulse creates the conditions for an obesity epidemic. Given that our world will likely continue changing at a rapid pace, are we doomed to have our impulses constantly playing catch up with our environment, and does that potentially doom us as a species?

SK: In fact, we’re not playing catch up; we’re stuck. For any evolutionary change to take place, the environment has to remain more or less constant for many generations, so that evolution can select the traits that are adaptive and eliminate those that are not. When the environment undergoes rapid change within the space of a generation or two, as it has been for the last couple of millennia, if not more, then evolution can’t happen because nature can’t determine which traits to select and which to eliminate. So they remain at a standstill. Our brain (and the rest of our body) are essentially frozen in time -- stuck in the Stone Age.

One example of this is that when we watch a scary movie, we get scared, and when we watch porn we get turned on. We cry when someone dies in a movie. Our brain cannot tell the difference between what’s simulated and what’s real, because this distinction didn’t exist in the Stone Age.

DC: One conclusion from your book is that we're something of a prisoner to our hard-wiring. Yes, there is some room for us to maneuver. But, in the end, our evolved nature takes over. If all of this holds true, is there room in our world for utopian (or even mildly optimistic) political movements that look to refashion how humans behave and interact with one another? Or does this science suggest that Edmund Burke was on to something?

SK: Steven Pinker, in his 2002 book The Blank Slate, makes a very convincing argument that all Utopian visions, whether they be motivated by left-wing ideology or right-wing ideology, are doomed to failure, because they all assume that human nature is malleable. Evolutionary psychologists have discovered that the human mind is not a blank slate, a tabula rasa; humans have innate biological nature as much as any other species does, and it is not malleable. Paul H. Rubin’s 2002 book Darwinian Politics: The Evolutionary Origin of Freedom gives an evolutionary psychological account of why Burke and classical liberals (who are today called libertarians) may have been right.

As a scientist, I am not interested in Utopian visions (or any other visions for society). But it seems to me that, if you want to change the world successfully, you cannot start from false premises. Any such attempt is bound to fail. If you build a house on top of a lake on the assumption that water is solid, it will inevitably collapse and sink to the bottom of the lake, but if you recognize the fluid nature of water, you can build a successful houseboat. A houseboat may not be as good as a genuine house built on ground, but it’s better than a collapsed house on the bottom of the lake. A vision for society based on an evolutionary psychological understanding of human nature at least has a fighting chance, which is a much better than any Utopian vision based on the assumption that human nature is infinitely malleable.

DC: So give us a hint. Why do beautiful people actually have more daughters?

SK: The basic idea is this: Whenever parents have genetic traits they can pass on to their children that are more valuable for boys than for girls, then they have more sons than daughters. Conversely, whenever parents have genetic traits they can pass on to their children that are more valuable for girls than for boys, then they have more daughters than sons. Physical attractiveness -- being beautiful -- is good for both boys and girls, but it’s much more advantageous for girls. Physical attractiveness of a woman is one of the most important considerations for men when they select both long-term and short-term mates, but a man’s physical attractiveness is important for women only when she’s looking for short-term mates. Women like to have affairs with good-looking men, but they don’t necessarily want to marry them, unless of course they are also rich and powerful.

So beautiful daughters will be more likely to take full advantage of their physical attractiveness than beautiful sons. Beautiful daughters are more likely to pass on their genes successfully to the next generation than beautiful sons, because they are more likely to find themselves in stable marriages to desirable spouses. In a representative sample of 3,000 young Americans, those who are “very attractive” had 36% greater odds of having a daughter compared to everyone else. Similarly, studies have found that big and tall parents are more likely to have sons, and short and thin parents are more likely to have daughters, because body size is more of an advantage to men than to women. Women are attracted to big and tall men much more than men are attracted to big and tall women.

DC: In the book you debunk the notion of “midlife crisis.” Why?

SK: We don’t debunk its existence; we believe it exists. But we suggest that it might exist for different reasons than people think. Midlife crisis is a mystery for evolutionary psychology, because there is really no reason for middle-aged men to change their behavior suddenly when they reach middle age. So we speculate in the book that middle-aged men may engage in a constellation of behavior which we associate with the phrase “midlife crisis,” not because they are middle-aged, but because their wives are. When their wives reach menopause, it means that, not only is the wife’s reproductive career over, but so is the husband’s, unless, of course, he can find a younger mate to replace (or, as often happened throughout evolutionary history, add to, since humans are naturally polygynous) the menopausal wife. We believe that “midlife crisis” might be a reflection of middle-aged men’s attempt to attract younger women because their wives are no longer reproductive. So we hypothesize that a 50-year-old man married to a 25-year-old wife will not undergo midlife crisis, whereas a (very rare) 25-year-old man married to a 50-year-old wife will. And, of course, evolutionary psychology can explain why there are very few young men married to middle-aged women. If you want to know, you have to read the book!

DC: Finally, what are some of the remaining mysteries in evolutionary psychology? Are there things that you still don’t know, questions for which you still don’t have answers?

SK: There are many questions for which we don’t yet have answers. We devote our last chapter to discussing some of these questions. For example, why do most middle-class people in western industrial nations have so few children? Most middle-class Americans can easily raise five or six children, and feed, clothe, and shelter them all very well. Yet most couples only want (and have) two children. This is a mystery for evolutionary psychology.

A related mystery is the fact that there seems to be a genetic transmission of fertility from parents to children, so that parents who have many siblings tend also to have many children themselves. This makes absolutely no sense from an evolutionary psychological perspective. If your parents had many children, that means you have many brothers and sisters who carry some of your genes, so you can afford not to have many children yourself. Conversely, if you are an only child, nobody else besides you carries half of your genes, so you have to have many children to spread your genes, to compensate for your parents’ lack of reproductive success. So there should really be a negative correlation between your parents' fertility and your own, but all the demographic studies show that the correlation is positive. Only children tend to beget only children. This is a mystery.