Do humans behave altruistically towards family members to help to ensure that their common genes make it to the next generation? And are humans somehow wired to prevent incest? Two pioneering, seemingly opposing theories propose that families treat kin better to pass on genes but avoid reproducing with each other, suggesting that the brain must have a means of detecting those to whom an individual is genetically related. John Tooby, of the University of California in Santa Barbara, has come up with a possible mechanism (see page 727).

According to work by William Hamilton and George Williams in the 1950s and 1960s, if a gene prompts behaviour that enhances the fitness of relatives while lowering that of the individual displaying the behaviour, it may still increase in frequency, because relatives often carry the same genes. Thus, the enhanced fitness of relatives can compensate for the fitness loss incurred by the individuals exhibiting the behaviour.

It was known even earlier that mating among family members lowers the fitness of offspring. In 1891, Finnish sociologist Edward Westermarck proposed that living together as children prevents sexual attraction among siblings, underpinning the 'incest taboo' in human societies. But the idea was ridiculed by Sigmund Freud, who held that the earliest expressions of childhood sexuality have strong elements of incestuous behaviour. Thus, the incest taboo, said Freud, existed to counteract a strong natural inclination. Tooby and his colleagues provide, for the first time, evidence of a biological mechanism by which humans detect their kin.

A handful of anthropological studies made during the past 50 years showing that unrelated children raised together are less likely to marry each other than are children raised separately and, when they are forced to marry each other, have fewer children, seemed to bolster Westermarck's hypothesis. “These studies were limited to indirect measures of sexual attraction and did not look at relatives,” explains Tooby. “We wanted to have a more natural, direct measure of behaviour.” As an evolutionary psychologist, Tooby also wanted to know how the mind figures out who is a relative and who is not.

He first pondered the question during his graduate studies at Harvard University in Massachusetts during the 1970s, but couldn't work on the topic until the mid-1990s. “There is no funding support for these kinds of studies,” says Tooby, who conducted the study with his wife and colleague Leda Cosmides and collaborator Debra Lieberman.

Their idea was that there must be ancestrally valid cues in the environment that allow the human brain to determine whether someone is related to them. One cue, they reasoned, would be seeing one's mother care for another infant. Younger siblings would have to rely on a different cue: the length of time spent living with another person. This would generally have been true for humans' forager ancestors, Tooby observes. “When food supplies were scarce they would break up into small groups to search for food,” he says. “A mother and her children would generally be in the same group.”

The researchers asked more than 600 people questions relevant to these two cues. They then tested how the cues correlated with two different motivational outputs: sibling altruism and disgust at the prospect of incest. For older siblings, exposure to their mother caring for their infant sibling caused altruistic motivation and sexual aversion. For younger siblings, time spent living with the older sibling had the same effects.

“The conclusion is that there are internal monitoring systems in the brain for these two cues that evolved over time,” explains Tooby. In turn, this monitoring system functions to regulate altruism and sexual attraction. “We found that it was not our subjects' beliefs about who was and was not a genetic relative that generated sexual disgust and altruism, but rather their exposure to these two cues.”