Part of the human brain is dedicated to detecting cheats, say
evolutionary psychologists, after a study with a brain-damaged
"We think it develops in all normal individuals, and that it
develops in part because our brains were selected to develop this
competence," says John Tooby at the University of California, Santa
Tooby and his colleagues studied a man who suffered accidental
damage to the limbic system, a brain region involved in processing
emotional and social information. RM, as he is referred to,
performed as well as other people on one set of reasoning problems,
did much worse on problems specifically designed to test reasoning
about social exchanges.
At its simplest, social exchange runs along the lines of "you
scratch my back and I'll scratch yours". Previous work has shown
that people, and some animals, are extremely good at keeping a check
of who owes who within a group - and at spotting and punishing
Researchers had proposed that general reasoning abilities could
account for this. But RM's deficit suggests that detecting social
cheaters depends on specialised neural circuitry, the team says.
Their conclusion is "robust," says Nigel Nicholson, an
evolutionary psychologist and director of the Centre for
Organisational Research the London Business School. "It's essential
we have trusting relationships with people in communities where we
are highly interdependent for survival and reproduction. Cheat
detection is very important," he adds.
The first problems given to RM and the 37 non-brain-damaged
controls concerned so-called precaution rules. For example: "If you
work with toxic chemicals, you have to wear a safety mask." The
second tested social contracts, for example: "If you go canoeing on
the lake, you have to have a clean bunk house."
RM recorded a score of 70 per cent on the precaution rule tests -
the same as the controls. But he scored only 39 per cent on the
social contract tests, compared with 70 per cent for the non-brain
Identical tests on two other people with brain damage similar to
BM's, but with a slightly different pattern of damage, showed that
their social contract reasoning was unimpaired.
"RM's differential impairment indicates that being able to detect
potential cheaters may be a separable component of the human mind,"
the researchers conclude in the journal Proceedings of the
National Academy of Sciences.
However, if a region of the brain has evolved to specialise in
cheat detection, it should be present in all people, the team
reasoned. Most experiments are performed on people living in modern,
So they studied people living in traditional, non-developed
communities in the Amazonian region of Ecuador. And they found that
these people were equally proficient at social exchange tasks, even
when the problems concerned social rules that were unfamiliar to
"What is quite amazing about their performance on cheater
detection is that it flies in the face of all ordinary ideas about
learning a higher level cognitive skill," Tooby told New
Scientist. "People are just as good at utterly unfamiliar rules
as they are with rules that are personally and culturally highly
Journal reference: Proceedings of the National Academy of
Sciences (DOI: 10.1073/pnas.122352699 and