The evolution of family homicide.

The evolution of family homicide

Evoluationary theories of social behavior -- sometimes lumped under the heading of sociobiology -- maintain that the appetites, aversions, motives, emotions and thinking patterns of humans, as well as other species, are shaped over the millennia to produce "nepotistic" social action. In other words, individual members of a species engage in typical actions to promote the survival and reproductive success of genetic relatives. Genetic relatedness is said to be linked to enhanced cooperation and reduced conflict between individuals.

How, then, do evolutionary theorists explain the tragic occurrence of murders within families? Recent statistical analyses of family homicides in the United States, Canada and elsewhere do, in fact, support evolutionary models of human behavior, say psychologists Martin Daly and Margo Wilson of McMaster University in Hamilton, Ontario.

Most family homicides involve one spouse killing the other, usually fueled by male attempts to control female sexual and reproductive behavior, the researchers report in the Oct. 28 SCIENCE. In the savanna and tropical environments that fostered the nonindustrial societies typical of most of human evolution, male competition for fertile women and the guarding of mates served useful purposes, particularly to ensure accurate paternity, Daly and Wilson contend.

That behavioral tendency can go awry in modern society, however. Most North American spouse-killers say the husband's concern with his wife's fidelity or her intention to end the marriage led him to initiate the violence, the researchers note. "Men strive to control women by various means and with variable success, while women strive to resist coercion and maintain their choices," they say. "There is brinkmanship in any such contest, and homicides by spouses of either sex m ay be considered the slips in this dangerous game."

Evolultionary influences also contribute to parent-child murders, Daly and Wilson add. The great majoirty of infanticide cases in nonindustrial societies reflect three instances in which parents withdraw affection from newborns: doubt that the child is the parent's own, conviction that the child is weak and unlikely to produce offspring as an adult, and external pressures (such as food scarcity and overburdening demands of older siblings) limiting a child's survival chances.

People apparently come to cherish their children increasingly over the years as the child's reproductive chances increase, the researchers say. Thus, parents in modern societies should be less likely to to kill offspring nearer to maturity (and reproductive success) and more likely to kill offspring in the first year after birth. Furthermore, becuase children impose more restrictions on mothers, infanticide should be more frequent among women. Canadian data on 845 child homicide victims between 1974 and 1983 bear out these predictions, Daly and Wilson assert.

An emphasis on genetic relatedness also aligns with the sad fact that children in stepparent families are disproportionately abused and killed in industrial nations, the investigators say. Data from the United States and Canada show the risks of homicide are greatest for youngsters 2 years old and younger. Abusive stepparents usually spare their own children in the same household, Daly and Wilson add.

Most stepparents are supportive, they acknowledge. "The fact remains that step-relationships lack the deep commonality of interest of the natural parent-offspring relationship, and feelings of affection and commitment are correspondingly shallowers. Differential rates of violence are one result."

The psychologists conclude that homicide -- in or out of the family -- is a rare, extreme outcome of social motives and self-interests that have been selected through evolution to produce adaptive behavior on average, not in all situations.