WHY CANʹT A WOMAN be more like a man?ʺ wailed Professor Henry Higgins inMy Fair Lady, the musical derived from George Bernard ShawʹsPygmalion. Certainly in many societies, across time, there have been women who were ʺmore like a man.ʺ Think of Joan of Arc, who was burned at the stake (on the minor charge of wearing menʹs clothing); or of George Sand, of whom Elizabeth Browning said, ʺYou are such a large-brained woman and a large-hearted man.ʺ Yet in part, we remember such womenbecausethey are singular, whether we envy their ability to break free or imagine...

ALICE HAD some trouble following the Red Queenʹs logic, that one has to run as fast as one can just to stay in place because everything else in the landscape is running as well. Biologists, however, find the image an apt one. Consider Matt Ridleyʹs engaging book on the origins of sexual selection, which he chose to callThe Red Queenin recognition of the problem that the sexes continually change each otherʹs costs and benefits. In a way, much of biology is a record of such selective arms races.¹ Ecology is rife with examples: if faster rabbits escape coyotes,...

A FAVORITE CARTOON of mine shows two deer, a buck and a pretty annoyed-looking doe standing on a hillside. The buck is tilting his head, saying, ʺSo I like rutting—so sue me.ʺ In the genetic gambling casino, success depends not only on individual strengths and weaknesses, but on environmental conditions, and whether or not there are groups to contend with. The buck, however, highlights an influence on all sexual species: from fish to flying squirrels, from Hanuman langurs to humans, males and females of most species experience different costs and benefits in reproducing, and these differences influence both lifetimes...

WE MUST REMEMBER that humans are primates, not deer or seals. We are complicated and diverse; ecological, social, and historical conditions can all contribute to the patterns we observe. Yet there are real regularities to how these forces interact, and sometimes we can make better sense of complex, apparently eclectic happenings through the selection lens. Consider the Salem witchcraft trials (more in chapter 10), during which Katherine Harrison was first immune from accusations, then targeted, then became safe once again through a powerful male accuserʹs turnabout to protector; self-interest and power differentials clearly had influence. But the basic relationship between...

MEN AND WOMEN are not all that different in size or appearance; no naive biologist would take two human specimens, male and female, and think they were different species (as has happened among some bird species). In general, male humans, regardless of current marriage system, are slightly larger than females, consistent with our evolutionary history of mild polygyny. This is because much, perhaps most, male-male competition in humans is not a matter of size, but of other traits: wealth, political savvy, and so on—traits that help in complex social competition more than sheer size.

As we saw in the...

A GREAT PHILOSOPHER never married, and, in a possibly apocryphal story I read as a child, on his deathbed he called for all of his works to be set upon his lap. When the works had been brought, he sighed, ʺAll of this is less than the weight of one grandchild.ʺ (And, to complete the story tidily, he promptly died.) His insight is an important one: What are resources for, anyway, if not to build our families? As we move through the stages of our lives, our struggle for resources never ceases; in fact, our very lifetimes are shaped by...

Men and women seek and use resources for reproductive success, but they can differ as much as peacocks and peahens in how they seek resources, what kinds of resources they seek, and how they use those resources. The reproductive ecology of the two sexes in humans, as in other mammals, creates opportunities for quite different uses of resources in reproductive competition by males and females, and different strategies (e.g., coalitions) to get them. In earlier chapters, I explored how our background of mammalian sex differences interacts with ecological conditions to yield different mating and resource systems. Here I want to...

AN IMPORTANT QUESTION isexactly what, in any species or society, contributes to greater or lesser lifetime reproductive success. As we have seen, in most mammals, and in the majority of traditional human societies for which data exist, status, power, or resource control enhance lifetime reproductive success, especially for men. Menʹs reproductive variation in traditional societies arises mostly through differential polygyny—higher-status men can marry earlier and more often than other men, and they can marry younger women of higher reproductive value.

Does this pattern have any relevance today? Two phenomena make it likely that today we will not see...

WE STARTED with very simple and general hypotheses about how resources affect reproductive success, and why men and women typically have quite different resource strategies. But complexity has crept in: By the demographic transition society discussed in the last chapter, market forces, governmental rules, and societal mores clearly influenced menʹs and womenʹs costs and benefits in resource, mating, and marriage decisions. Simple hypotheses about selfish genes, favoring themselves and their copies in kin, canʹt explain what we see. The world may be nasty, but it is much nicer than selfish genes in solitary animals would make it. As biologist Richard...

KATHERINE HARRISON was a witch. After the death of her rich and powerful husband, she was accused of witchcraft, based on events occurring years before. When accused, she was a rich widow who chose not to remarry, and she had no sons to protect her property. She continued to be accused for years, and was even forced to leave her community, until, in 1670, her older daughter became engaged to a man from a powerful family. The future father-in-law, previously an attacker, became her protector, and the accusations ceased.

The demographic and economic particulars of witchcraft trials show a pattern...

COALITIONS, like so many other phenomena, can be a reproductive strategy; and if this is true, male and female coalitions will tend to be different. Among bottle-nosed dolphins, females often swim with each other. Dolphin society is (like chimpanzee groups and many traditional societies) a ʺfission-fusionʺ society, in which individuals travel and feed together, splitting up and reforming. But the sexes differ. A female might swim with her mother today, her mother and sister tomorrow, and her friends next week. Males, however, are a very different story. They travel in small and stable groups: you will always see the same...

HUMANS, like other primates, move almost imperceptibly from ʺcoalitionsʺ to ʺpolitics.ʺ We have come far from our starting consideration of the ecology of sex differences in reproduction. But a variety of current issues have precisely the characteristics that make this logical stretch worthwhile: simple rules, interacting with environmental and historical particulars, create outcomes of increasing scale and complexity. If we can understand how men and women profited reproductively from differing resource strategies in the past, perhaps we can follow the emergence of complexity and begin to make sense of likely differences—and how to separate those from issues of equity—...

THE PRUSSIAN miliary strategist Carol von Clausewitz said war was simply ʺthe continuation of state policy [politics] by other means.ʺ Although warfare gives rise to some of the strongest, most tightly knit and potent international coalitions in modern times, that is surely not its evolutionary context, for states are a relatively modern phenomenon, while organized intergroup conflict among competing coalitions is as old as, or older than, humanity itself. Conflicts of interest, if not coalitions in open aggression, are universal among living things, and certainly lethal conflict exists in many species. Thus it makes sense to begin our inquiry by...

IT IS NOT SURPRISING that the functional relationships of warfare are clearer in smaller, simple societies than in large politically complex ones. The transition from preindustrial warfare to the complex multinational warfare discussed in treatises on military history may seem almost unfathomable, but we must explore it if we are to understand whether modern warfare is functionally different from tribal warfare.

The military historian John Keegan’s description of Alexander the Great suggests that even in large hierarchical armies, as during Philip’s and Alexander’s rule in Macedonia, personal characteristics, kin-group size, and ability to inspire loyalty and reciprocity still were crucial...

IT IS EASY to imagine that our evolutionary past is remote, unconnected to our lives today, and of interest only when we think of traditional societies or ancient history. Yet, as we saw in the last few chapters, our evolved tendencies interact with todayʹs novel environments. Todayʹs cities, no less than yesterdayʹs rain forests and savannas,areour environments. Just as soldiers in modern warfare display remnants of behaviors from past times, so do we all in our daily lives, as we live, work, and raise our families. Some physical and social aspects of our current environments are evolutionarily novel,...