To take behavioral genes into account therefore seems a prudent step when assess- 5 ing human behavior. Sociobiology (or Darwinian anthropology, or evolutionary psychology, or whatever more politically acceptable term one chooses to call it) offers a key link in the attempt to explain the biological foundation of human nature. By asking questions framed in evolutionary theory, it has already steered research in anthropology and psychology in new directions. Its major research strategy in human studies has been to work from the first principles of population genetics and reproductive biology to predict the forms of social behavior that confer the greatest Darwinian fitness. The predictions are then tested with data taken from ethnographic archives and historical records, as well as from fresh field studies explicitly designed for the purpose. Some of the tests are conducted on preliterate and other traditional societies, whose conservative social practices are likely to resemble most closely those of Paleolithic ancestors. A very few societies in Australia, New Guinea, and South America in fact still have stone-age cultures, which is why anthropologists find them especially interesting. Other tests are conducted with data from modern societies, where fast-evolving cultural norms may no longer be optimally fit. In all these studies a full array of analytic techniques is brought to bear. They include multiple competing hypotheses, mathematical models, statistical analysis, and even the reconstruction of the histories of memes and cultural conventions by the same quantitative procedures used to trace the evolution of genes and species.
In the past quarter-century, human sociobiology has grown into a large and technically complex subject. Nevertheless, it is possible to reduce its primary evolutionary principles to some basic categories, which I will now briefly summarize.
Kin selection is the natural selection of genes based on their effects on individuals carrying them plus the effects the presence of the genes has on all the genetic relatives of the individuals, including parents, children, siblings, cousins, and others who still live and are capable either of reproducing or of affecting the reproduction of blood relatives. Kin selection is especially important in the origin of altruistic behavior. Consider two sisters, who share half their genes by virtue of having the same father and mother. One sacrifices her life, or at least remains childless, in order to help her sister. As a result the sister raises more than twice as many children as she would have otherwise. Since half of her genes are identical to those of her generous sister, the loss in genetic fitness is more than made up by the altruistic nature of the sacrifice. If such actions are predisposed by genes and occur commonly, the genes can spread through the population, even though they induce individuals to surrender personal advantage.
From this simple premise and elaborations of it have come a wealth of predictions about patterns of altruism, patriotism, ethnicity, inheritance rules, adoption practices, and infanticide. Many are novel, and most have held up well under testing.
Parental investment is behavior toward offspring that increases the fitness of the latter at the cost of the parent's ability to invest in other offspring. The different patterns of investment have consequences for the fitness of the genes that predispose individuals to select the patterns. Choose one, and you leave more offspring; choose another, and you leave fewer offspring. The idea has given rise to a biologically based "family theory," spinning off new insights on sex ratios, marriage contracts, parent-offspring conflict, grief at the loss of a child, child abuse, and infanticide. . . .
Mating strategy is influenced by the cardinal fact that women have more at stake in 10 sexual activity than men, because of the limited age span in which they can reproduce and the heavy investment required of them with each child conceived. One egg, to put the matter in elemental terms, is hugely more valuable than a single sperm, which must compete with millions of other sperm for the egg. The achievement of pregnancy closes off further breeding opportunity of the mother for a substantial fraction of her remaining reproductive life, whereas the father has the physical capacity to inseminate another woman almost immediately. With considerable success, the nuances of this concept have been used by scientists to predict patterns of mate choice and courtship, relative degrees of sexual permissiveness, paternity anxiety, treatment of women as resources, and polygyny (multiple wives, which in the past at least has been an accepted arrangement in three-quarters of societies around the world). The optimum sexual instinct of men, to put the matter in the now familiar formula of popular literature, is to be assertive and ruttish, while that of women is to be coy and selective. Men are expected to be more drawn than women to pornography and prostitution. And in courtship, men are predicted to stress exclusive sexual access and guarantees of paternity, while women consistently emphasize commitment of resources and material security.
Status is central to all complex mammal societies, humanity included. To say that people generally seek status, whether by rank, class, or wealth, is to sum up a large part of the catalogue of human social behavior. In traditional societies genetic fitness of individuals is generally but not universally correlated with status. In chief- doms and despotic states especially, dominant males have easy access to multiple women and produce more children, often in spectacular disproportion. Throughout history, despots (absolute rulers with arbitrary powers of life and death over their subjects) commanded access to hundreds or even thousands of women. Some states used explicit rules of distribution, as in Inca Peru, where by law petty chiefs were given seven women, governors of a hundred people eight, leaders of a thousand people fifteen, and lords and kings no fewer than seven hundred. Commoners took what was left over. The fathering of children was commensurately lopsided. In modern industrial states, the relationship between status and genetic fitness is more ambiguous. The data show that high male status is correlated with greater longevity and copulation with more women, but not necessarily the fathering of more children.
Territorial expansion and defense by tribes and their modern equivalents the nation states is a cultural universal. The contribution to survival and future reproductive potential, especially of tribal leaders, is overwhelming, and so is the warlike imperative of tribal defense. "Our country!" declared Commodore Stephen Decatur, hard- fighting hero of the War of 1812, "may she always be right; but our country, right or wrong." (Personal aggressiveness has its Darwinian limits, however; Decatur was killed in a duel in 1820.)
Biologists have determined that territoriality is not unavoidable during social evolution. It is apparently entirely absent in many animal species. The territorial instinct arises during evolution when some vital resource serves as a "density-dependent factor." That is, the growth of population density is slowed incrementally by an increasing shortage of food, water, nest sites, or the entire local terrain available to individuals searching for these resources. Death rates increase or birth rates decrease, or both, until the two rates come more or less into balance and population density levels off. Under such circumstances animal species tend to evolve territorial behavior. The theoretical explanation is that individuals hereditarily predisposed to defend private resources for themselves and their social group pass more genes on to the next generation.
In contrast, the growth of other species is not leveled off by limiting resources but by rising amounts of emigration, disease, or predation. When such alternative density- dependent factors are paramount, and resource control is therefore not required, territorial defense usually does not evolve as a hereditary response.