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Johnstone Family Professor, Department of Psychology; Harvard University; Author, The Better Angels of Our Nature

Evolutionary Genetics Explains The Conflicts of Human Social Life

Complex life­ is a product of natural selection, which is driven by competition among replicators. The outcome depends on which replicators best mobilize the energy and materials necessary to copy themselves, and on how rapidly they can make copies which can replicate in turn. The first aspect of the competition may be called survival, metabolism, or somatic effort; the second replication or reproductive effort. Life at every scale, from RNA and DNA to whole organisms, implements features that execute—and constantly trade off­—these two functions.

Among life's tradeoffs is whether to allocate resources (energy, food, risk, time) to pumping out as many offspring as possible and letting them fend for themselves, or to eking out fewer descendants and enhancing the chances of survival and reproduction of each one. The continuum represents the degree of parental investment expended by an organism.

Since parental investment is finite, investing organisms face a second tradeoff, between investing resources in a given offspring and conserving those resources to invest in its existing or potential siblings.

Because of the essential difference between the sexes—females produce fewer but more expensive gametes—the females of most species invest more in offspring than the males, whose investment is often close to zero. Mammalian females in particular have opted for massive investment, starting with internal gestation and lactation. In some species, including Homo sapiens, the males may invest, too, though less than the females.

Natural selection favors the allocation of resources not just from parents to offspring but among genetic kin such as siblings and cousins. Just as a gene that encourages a parent to invest in offspring will be favoring a copy of itself that sits inside those offspring, so a gene that encourage an organism to invest in a brother or cousin will, some proportion of the time, be helping a copy of itself, and will be selected in proportion to the benefits conferred, the costs incurred, and the degree of genetic relatedness.

I've just reviewed the fundamental features of life on earth (and possibly life everywhere), with the barest mention of contingent facts about our own species: only that we're mammals with male parental investment. I'll add a second: that we're a brainy species that deals with life's conundrums not just with fixed adaptations selected over evolutionary time, but with facultative adaptations (cognition, language, socialization) that we deploy in our lifetimes and whose products we share via culture.

From these deep principles about the nature of the evolutionary process, one can deduce a vast amount about the social life of our species. (Credit where it's due: William Hamilton, George Williams, Robert Trivers, Donald Symons, Richard Alexander, Martin Daly, Margo Wilson.)

•     Conflict is a part of the human condition. Notwithstanding religious myths of Eden, romantic images of noble savages, utopian dreams of perfect harmony, and gluey metaphors like attachment, bonding, and cohesion, human life is never free of friction. All societies have some degree of differential prestige and status, inequality of power and wealth, punishment, sexual regulations, sexual jealousy, hostility to other groups, and conflict within the group, including violence, rape, and homicide. Our cognitive and moral obsessions track these conflicts. There are a small number of plots in the world's fiction, and are defined by adversaries (often murderous), by tragedies of kinship or love, or both. In the real world, our life stories are largely stories of conflict: the hurts, guilts, and rivalries inflicted by friends, relatives, and competitors.

•    The main refuge from this conflict is the family—collections of individuals with an evolutionary interest in one another's flourishing. Thus we find that traditional societies are organized around kinship, and that political leaders, from great emperors to tinpot tyrants, seek to transfer power to their offspring. Extreme forms of altruism, such as donating an organ or making a risky loan, are typically offered to relatives, as are bequests of wealth after death—a major cause of economic inequality. Nepotism constantly threatens social institutions such as religions, governments, and businesses that compete with the instinctive bonds of family.

•     Even families are not perfect havens from conflict, because the solidarity from shared genes must contend with competition over parental investment. Parents have to apportion their investment across all their children, born and unborn, with every offspring equally valuable (all else being equal). But while an offspring has an interest in its siblings' welfare, since it shares half its genes with each full sib, it shares all of its genes with itself, so it has a disproportionate interest in its own welfare. The implicit conflict plays itself out throughout the lifespan: in postpartum depression, infanticide, cuteness, weaning, brattiness, tantrums, sibling rivalry, and struggles over inheritance.

•     Sex is not entirely a pastime of mutual pleasure between consenting adults. That is because the different minimal parental investment of men and women translates into differences in their ultimate evolutionary interests. Men but not women can multiply their reproductive output with multiple partners. Men are more vulnerable than women to infidelity. Women are more vulnerable than men to desertion. Sex therefore takes place in the shadow of exploitation, illegitimacy, jealousy, spousal abuse, cuckoldry, desertion, harassment, and rape.

•     Love is not all you need, and does not make the world go round. Marriage does offer the couple the theoretical possibility of a perfect overlap of genetic interest, and hence an opportunity for the bliss that we associate with romantic love, because their genetic fates are bound together in the same package, namely their children. Unfortunately those interests can diverge because of infidelity, stepchildren, in-laws, or age differences­­—which are, not coincidentally, major sources of marital strife.

None of this implies that people are robots controlled by their genes, that complex traits are determined by single genes, that people may be morally excused for fighting, raping, or philandering, that people try to have as many babies as possible, or that people are impervious to influences from their culture (to take some of the common misunderstandings of evolutionary explanations). What it does mean is that a large number of recurring forms of human conflict fall out of a small number of features of the process that made life possible.