Many aspects of human behavior have been demystified by taking seriously the possibility that evolution works on minds as well as bodies, and the resulting Darwinian analyses have laid bare many previously hidden dimensions of our psychology. For example, in a pre-Darwinian world who would have guessed that women prefer different kinds of men depending on where they are in their menstrual cycles? These numerous discoveries have sprung from a simple but powerful approach: Select a specific challenge that regularly confronted ancestral humans and ask what mental algorithm would have solved it.
While this bottom-up, challenge-to-algorithm approach has been very successful, it won't answer every kind of question about the mind. Sometimes, instead of trying to predict the mind's inclinations and biases from the challenges it habitually faced, we're confronted with a conspicuous piece of human psychology whose purpose is obscure. To decipher these puzzling pieces of mental machinery we need a top-down approach; we need to unearth the evolutionary pressures that shaped them. This is precisely the terrain where Will Lassek and I first met. As a perpetually curious former Assistant Surgeon General, Will had decided to audit my freshman "Sex and Evolution" class. How serendipitous!
The puzzling piece of human psychology that captured our joint attention was men's fascination with women's body shape. Of course (heterosexual) men evolved to admire and compare women; after all, women are their potential mates, potential mothers of their children. Men whose preferences steered them to the best mothers would pass on their mating preferences to future generations. No mystery there; that's how evolution works. But why should fat, and fat distribution, be a human male preoccupation? Evidence from all over the world suggests that men strongly prefer women who have a lot of body fat (roughly 30 percent of their body weight) and whose body fat is distributed in a particular way, with very little in the waist but much more in the hips, buttocks and thighs, producing a small waist-hip ratio. Why have men evolved to prefer such high levels of fat--more than bears settling down to hibernate or whales swimming in frigid waters? And even if it could be explained how more fat makes a better mom, why would it matter where she stored it; what is the message contained in a low waist-hip ratio? So this male preference actually consists of two nested psychological puzzles.
The standard line of thinking is that mating proclivities should track reproductive benefits: We evolve to prefer trait X because potential mates who have more of trait X will give us more or better kids. Let's see where that leads. Pregnancy and nursing are calorically expensive for women, and fat has a lot of calories. A promising idea perhaps, but a minute's refection shows that this same argument would apply to all of the several thousand mammal species on the planet. Unfortunately for this hypothesis most of them have very little fat; on a percentage basis, a typical slender young woman has roughly six times as much fat as the average female mammal. If moms of every species have to pay the caloric costs of making babies, why would women need so much more fat than monkey or meerkat moms? Nor can the energy costs of motherhood explain why the location of the fat matters. If it's all about the calories waist fat should be just as good as hip fat, but male psychology says they're not at all equivalent.
Digging farther down, we discovered that it's apparently not about the calories. Bodies run on calories but they are made out of raw materials, of many different kinds. Thinking about fat as a specific kind of building material rather than an energy source draws one's attention to the brain which, neglecting water, is mostly fat. The reason this is a promising idea is that humans have extraordinarily large brains, on the order of six or seven times larger than expected for a mammal of our size. Is it just a coincidence that humans have brains six times larger than similarly sized mammals, and that women have six times as much fat as typical mammals? Probably not. In a species where very large brains are the norm, females would need a substantial "fat account" to build the brains of their offspring. This potentially explains why women have such a high level of body fat compared to their mammalian cousins.
But the lynchpin of this story focuses on the second of the two nested puzzles: the unique distribution of women's fat. Remember, men like women with substantial body fat but strongly prefer that fat to be in the hips and thighs, not in the waist. If women store fat to build their infants' brains then we need to know what kinds of fat those brains are made of. The rarest and most important brain fat is an omega-3 fat called DHA, and women systematically store the DHA they accumulate in their hips and thighs and then draw it down during pregnancy and nursing. Because omega-3 fats must come from the diet, they may often be in short supply. So, the "mysterious" male mating preference tracks women's ability to build big-brained kids. This kind of male preference would have been very beneficial in a species like humans where brain size was evolving rapidly.
This research satisfyingly connects several themes in the study of human evolution, but the arc of discovery leads even farther, into the realm of public health. A huge shift in US food policy--driven by a collaboration of poor science and corporate interests--has progressively sucked the DHA out of the American food supply and replaced it with metabolically competing omega-6 fats. This means that American women are coming up short on the raw materials they need to nourish their infants' neural development. For example, American women have very low levels of DHA in their breast milk, less than half that of the tribal Amazonian women we recently studied with an interdisciplinary team led by Melanie Martin of U.C. Santa Barbara's Anthropology Department. If American women have a low percentage of DHA in their fat, their bodies may also be driving them to store more pounds of fat in an effort to have enough DHA for their infants' brains.
By taking an evolutionary approach to a well-known but poorly understood fact about male psychology we came to a much better understanding of women's reproductive physiology and flagged a potential crisis looming in the American diet. Who says evolution is all about the past? This wide-ranging research is summarized in our current book, Why Women Need Fat.
Written by Steven Gaulin and Will Lassek