Why do we want what we want?

A list in London

In the summer of 1838, probably in July, a twenty-nine-year-old naturalist in London took a sheet of paper and wrote two headings: “Marry” and “Not marry.” Charles Darwin had been home from the voyage of the Beagle for less than two years, and his private notebooks were already full of the ideas that would become the theory of natural selection. But here was a question the notebooks could not answer, so he did what a scientist does and listed the evidence. Under “Marry” went children, “if it please God,” and a constant companion in old age, someone to be loved and played with, “better than a dog anyhow.” Under “Not marry” went less money for books, a terrible loss of time, and less freedom to go where he liked. He added it up, and at the foot of the page he wrote his verdict: “Marry—Marry—Marry. Q.E.D.” He had no one in particular in mind yet. In November he proposed, at Maer in Staffordshire, to his cousin Emma Wedgwood, and they were married there on 29 January 1839.

It is a charming document, and a slightly comic one: the man who would explain the origin of species settled the biggest decision of his life with a pros-and-cons list. But look at what is on the list. Companionship, children, a home. Nobody taught Darwin to want these things, and he would have found it hard to say why he did. This course is about the wants that sit behind lists like his, and about a young science, evolutionary psychology, that tries to explain where they come from. It is also about how much to trust that science, because it is one of the most promising and most disputed fields in psychology.

Two kinds of “why”

Darwin showed that natural selection shaped the bodies of living things. Features that helped their owners survive and leave offspring became common; features that did not faded away. That is why we have eyes, wings, and hands. Evolutionary psychology is the idea that selection shaped minds too. If hunger and fear have an evolutionary history, why not attraction, jealousy, and a parent’s love? The label was used as early as 1973 by the biologist Michael Ghiselin, and it became the banner of a research program through a 1992 collection of essays, The Adapted Mind, edited by Jerome Barkow, Leda Cosmides, and John Tooby. It grew out of sociobiology, a name made famous by Edward O. Wilson’s 1975 book.

To see what the idea claims, and what it does not, separate two questions that sound alike. The Dutch-British biologist Niko Tinbergen pointed out in 1963 that there are four ways to ask “why” about any behavior: about its mechanism (what produces it right now?), its development in one life, its function (what is it good for?), and its evolution over generations. The American biologist Ernst Mayr had earlier folded these into two, proximate and ultimate causes. Take a sweet tooth. The proximate answer is that sugar excites the reward circuits of the brain. The ultimate answer is that for ancestors who lived among wild fruit, with no supermarkets, a liking for sweetness helped them find energy. Both answers are true, and neither replaces the other.

Notice what this does not claim. Nobody has to know why they want something. A couple in a café are not thinking about babies; they are enjoying each other’s company. In this view, evolution installed the wanting, and a person’s reasons are another matter altogether. Contraception shows how loose the link can be: people have sex for pleasure and closeness, often take care that no child results, and the wanting keeps its hold anyway.

Are we vehicles for our genes?

You will often meet a vivid way of putting all this. The Victorian writer Samuel Butler remarked in 1877 that a hen is only an egg’s way of making another egg. A century later biologists said it seriously. Wilson, quoting Butler, wrote that the organism is only DNA’s way of making more DNA, and Richard Dawkins’s The Selfish Gene (1976) described bodies as “survival machines” built by genes.

There is something right in this picture. Think of your ancestors, back and back: every one of them, without exception, had a child who lived long enough to have children. Traits that made that less likely were passed on less often. So the wants we inherited are, on the whole, ones that did not get in the way. But the vivid phrase invites three mistakes.

Genes do not make plans. When biologists call a gene “selfish,” they mean something plain: copies of genes that help their bodies reproduce become more common. Dawkins himself stressed that genes have no foresight and no goals. “Selfish” is a metaphor for a statistical fact.

“Shaped for” is not “meant for.” Evolution can help explain where your wants came from. It cannot tell you what your life is for. Many full and admirable lives contain no children, and nothing in biology calls that a failure.

People do not try to maximize their offspring. If they did, richer societies would have more children. In general the reverse has happened: fertility has fallen as countries have grown richer. (Within one society the link between wealth and family size is mixed; the claim is about the pattern across countries and over time.) In 2023 South Korea’s total fertility rate was 0.72 children per woman, China’s was about 1.0 by United Nations estimates (China publishes no routine official figure), and the United States’ was about 1.6, against the roughly 2.1 needed to keep a population steady. Evolution installs wanting, not goals: people want closeness, comfort, and a child to love, and nobody wants “more copies of my genes.” When the world changes faster than the wants do, the old wants keep pulling in a new setting.

Even the picture of the body as a temporary vehicle needs care. You pass on half of your genes to a child, a quarter to a grandchild, and an eighth to a great-grandchild. Ten generations on, the expected share of any one ancestor’s genes is about one part in a thousand. Genes do not make a person immortal. What lasts is information, copied, reshuffled, and mostly forgotten.

Hunters, gatherers, and a good story

Here is the kind of argument that made the field famous, and gave it its critics. Our ancestors divided the work, so the story goes: men hunted and women gathered food and watched the children. Hunting rewards a single target held in the mind while distractions are ignored. Gathering and child-minding reward scanning many things at once: the ripe fruit, the child by the rock, the snake in the grass. Natural selection therefore gave men a more focused attention and women a more divided, multitasking one. Today, men are said to be single-minded and women good at doing several things at once.

It sounds sensible. Is it true? Test each link in turn, starting with the facts to be explained.

Do women multitask better than men? The evidence is mixed. A few studies find a small advantage for women on some tasks, many find none, and a few find an advantage for men; no large review has settled it, and “women multitask better” is not an established fact. What is better established are two other averages. In tests of mental rotation, which ask you to imagine turning a 3-D shape, men score higher on average, by roughly half a standard deviation (0.56 in a large 1995 review by Daniel Voyer and colleagues). And in tests of remembering where objects were placed, women do slightly better on average; Irwin Silverman and Marion Eals proposed in 1992 that this came from gathering. In a 2007 study of internet data from 40 countries, men beat women on mental rotation in all 40, and women beat men on object location in 35. These are real averages. The gathering explanation, though, is a hypothesis, and the question is how much the averages tell you about any person.

The psychologist Janet Hyde looked at that question in 2005. She gathered 46 reviews that had combined the results of many studies of psychological differences between women and men, a total of 124 measured differences, and found that 78 percent were small or close to zero. A few were large, such as how fast and how far people throw. A later survey of 106 reviews (Zell and colleagues, 2015) reached much the same average. For most traits, the two groups overlap almost completely. Try it below.

The second link in the story is the division of labor itself. “Man the hunter, woman the gatherer” is less tidy than it sounds. A 2023 survey of the ethnographic record by Abby Anderson and colleagues found women hunting in 50 of the 63 foraging societies that had explicit data, about four in five. Fifteen scholars of hunter-gatherers answered in 2024 that the sample was skewed and some coding mistaken, and that “hunting” in the survey included small game; men still do most of the big-game hunting. A teenager buried with hunting tools in the Peruvian Andes about 9,000 years ago was identified as female by Randall Haas and colleagues in 2020, though one burial cannot show how common that was. The honest summary is that women hunt in many societies, men do more of the hunting for large game, and the division of labor is a tendency, not a law. The first link of the story, “men hunted, women did not,” is too tidy.

The third link is the most important. Almost any pattern can be given a Stone Age explanation. If women had turned out to be better at focusing, a story could have been told about the patience gathering requires; if men had turned out to be better at multitasking, one about the many threats a hunter must watch. In 1979 the biologists Stephen Jay Gould and Richard Lewontin called such explanations “just-so stories,” after Rudyard Kipling’s tales of how the leopard got his spots. The test of an evolutionary explanation is not whether it is plausible, which it nearly always is, but whether it predicts something that could have turned out otherwise, and then does.

Five questions to ask of any claim about human nature

This is the toolkit the rest of the course will use. When you read that evolution explains some feature of human nature, ask:

1. Was it measured? Is there a study, or only a story about the Stone Age?
2. How big is it? How much do the two groups overlap, and what does the difference say about one person?
3. Does it hold in many societies? Most psychology has studied a narrow slice of humanity. A 2010 review by Joseph Henrich, Steven Heine, and Ara Norenzayan found that in leading psychology journals from 2003 to 2007, about 96 percent of the participants came from Western industrialized countries, home to about 12 percent of the world’s people. They called such samples “WEIRD”: Western, educated, industrialized, rich, and democratic.
4. Has it been replicated? When 270 researchers tried in 2015 to repeat 100 published psychology experiments, about 36 percent of the repeats found a statistically significant result, against 97 percent of the originals, and the average effect was about half the size. Some findings about mating have held up well, as you will see in Lesson 3, and some have not.
5. Could the explanation have been wrong? What would it forbid? An explanation that fits every outcome explains none.