By Andrea Estrada

A study involving more than 600 test subjects provides evidence that the human brain contains a nonconscious mechanism that identifies genetic siblings. It does so on the basis of the same cues that were reliable predictors of genetic relatedness among our hunter-gatherer ancestors.

The researchers at UCSB and the University of Hawaii, Honolulu, found that people felt more altruistic toward individuals whom this mechanism identified as siblings and, at the same time, felt more averse to engaging in sexual relations with them. Their findings will be published in the Feb. 15 issue of the science journal Nature.

“The old thinking was that Darwinism applied to humans physically, but not socially. Now we see the evolution of a mechanism that finely regulates important aspects of human social behavior,” said John Tooby, professor of anthropology and co-director of the Center for Evolutionary Psychology at UCSB. He completed the study with Leda Cosmides, professor of psychology and also co-director of the Center for Evolutionary Psychology, and Debra Lieberman, a former student who is now a professor of psychology at the University of Hawaii.

Mechanisms like the one identified in the current study have been found in many species, Tooby added, but their existence in humans had been a matter of controversy.

Fundamental theories in evolutionary biology have long proposed that biological kinship is the foundation of the family unit. It creates the sense of altruism that exists among genetically related family members, and is the source of taboos that forbid incestuous sexual relations. Until recently, questions have persisted regarding the existence of a mechanism that is responsible for establishing those behaviors.

The results of the study could also have implications for health care professionals, such as psychiatrists and psychologists, who treat victims of incest and those who commit it.

According to the researchers, the development of altruism between siblings is a result of natural selection, as are their aversions to sexual relations with one another and their aversion to sexual relations among siblings in general. The study’s findings indicate these sensibilities are not primarily a result of socialization by parents or peers, but of motivational systems that evolved to respond to cues of genetic relatedness.

The question the researchers sought to answer was how siblings recognize their close genetic matches. Drawing on the socioecology of ancestral human foragers, they found the answer in a set of cues that enable humans to identify their brothers and sisters as siblings. For older siblings, what the researchers refer to as “maternal perinatal association”—seeing their mothers care for infant siblings—activates the mechanism in the brain.

This cue is unavailable to younger siblings who didn’t see their mothers care for the older brothers and sister. For younger siblings, the mechanism is triggered by the amount of time, from infancy to near adulthood, that they live together as a family. The researchers found that duration of co-residence regulates sibling altruism and sexual aversion toward adopted and stepsiblings as well. “The cues regulate sibling altruism and sexual aversion, no matter what we believe,” Cosmides said.

The discovery of a mechanism designed to make family relationships non-erotic casts doubt on Dr. Sigmund Freud’s view that family members are the first and most powerful objects of sexual desire, say the authors. It also helps to settle a long-running debate in anthropology about whether family relationships are socially created purely by culture, or whether evolved mechanisms play a role.