UCSB Study on Sibling
Detection Mechanism Highlighted in 'Nature'
February 15, 2007
(Santa Barbara, Calif.) – Fundamental theories in
evolutionary biology have long proposed that biological kinship is
the foundation of the family unit. It not only creates the sense of
altruism that exists among genetically related family members, but
also establishes boundaries regarding sexual relations within the
nuclear family. Questions have persisted, however, regarding the
means by which humans recognize family members – particularly
siblings – as close genetic relatives.
A team of researchers at the University of California, Santa
Barbara, has found evidence of a nonconscious mechanism in the human
brain that identifies genetic siblings on the basis of cues that
guided our hunter-gatherer ancestors. Their findings will be
published in the February 15 issue of the science journal Nature. In
a study involving more than 600 test subjects, the researchers found
that people felt more altruistic toward individuals this mechanism
recognized as siblings, and, at the same time, felt a greater
aversion to engaging in incestuous sexual relations with them.
"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 at the center and now a professor
of psychology at the University of Hawaii. Mechanisms such as the
one identified in the current study have been found in many species,
he added, but their existence in humans had been a matter of
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, which, in
turn, increases feelings of both altruism and sexual aversion toward
younger brothers and sisters.
This cue, however, is unavailable to younger siblings whose birth
order precludes the opportunity of watching their mothers care for
older brothers and sisters. For these siblings, the mechanism is
triggered by the amount of time they live together as a family
during the period from the younger siblings' infancy through
adolescence. The researchers found that this "co-residence"
regulates sibling altruism and sexual aversion toward adopted and
step-siblings as well – individuals whom the subjects consciously
believe to be genetically unrelated. "This shows that the mechanism
operates independently of our beliefs about kinship," Cosmides said.
"The cues regulate sibling altruism and sexual aversion, no matter
what we believe."
The discovery of a mechanism designed to make family
relationships non-erotic casts doubt on 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 in
the brain play a role.
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.
"The theory gives a means of identifying who might be at risk,"
said Tooby. "Siblings who have lived separately for long periods of
time have not been exposed to the cues the brain uses to determine
who is a sibling. This may offer an explanation as to why someone
might have an inclination toward incest." It also suggests, he says,
ways of building families that would be more strongly and reliably
linked together by bonds of affection.