Incest and Human Kin Detection

Previously, I have written a post on the incest taboo, which can be found
here. My argument is that the moral abhorrence aroused by incest is one of the clearest examples of a moral judgment that can be explained as rooted in human evolutionary nature. We are naturally inclined to abhor sex with those with whom we have been raised, and this natural propensity was probably shaped in evolutionary history as a mechanism to avoid the deleterious effects of inbreeding. Edward Westermarck elaborated this Darwinian account of the incest taboo, and a wide range of evidence supports it.

Now we have an article in the February 15th issue of the British science journal Nature that adds more evidence and reasoning in favor of the Westermarck hypothesis. The article is Debra Lieberman, John Tooby, and Leda Cosmides, "The Architecture of Human Kin Selection." The abstract of the article can be found here.

At the University of California at Santa Barbara, Debra Lieberman conducted studies of over 600 students, measuring their reactions to sibling incest and sibling altruism. (I first met Debra in 1998 in Helsinki, Finland, at a conference on Westermarck; and I have been following her work ever since.) From these studies, she concludes that human beings have an evolved system for detecting genetic relatedness. Older children identify younger children as their siblings if the younger children have been associated with their mother from birth. Younger children identify older children as their siblings if they have lived together over a long time. Lieberman discovered that these kin detection cues were correlated with feeling disgust at sibling incest and feeling inclined to give altruistic help to siblings.

An interesting finding is that even when the students believed that their siblings were step-siblings or adoptive-siblings, they still felt a strong repugnance towards sibling incest and a strong propensity towards altruistic behavior with their siblings.

The assumption is that the human brain is endowed with a kin recognition mechanism that works through these two cues--association with one's mother in infancy and living together in childhood--to prompt the moral emotions of the incest taboo and sibling altruism.

There remain many gaps in this research. We don't know exactly the genetic basis for this or the precise neural mechanisms for it. But still this looks like a scientifically plausible account of how a natural moral judgment that is universal to all human societies has been shaped by our evolved human nature.