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Psychopathy Twins


Psychopathy Twins

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  • pg 1

Original sinners?
May 26th 2005
From The Economist print edition

Evidence that psychopaths are born, not made

RESEARCHERS at the Institute of Psychiatry, in London, are not shy about tackling
controversial topics. One of them, Terrie Moffitt, was responsible for studies that
showed how different versions of the gene for one of the brain's enzymes resulted in
different predispositions to criminal activity. Another, Robert Plomin, found the first
plausible candidate for a gene that boosts intelligence. Now, Dr Moffitt and Dr Plomin
have been helping two other researchers, Essi Viding and James Blair, with an
equally high-profile study—one which asks whether psychopaths are born that way,
or are made so by their upbringings.

That, of course, is rather a crude way of putting it. After decades of debate,
biologists have come to understand what was blindingly obvious to most laymen—
which is that rather than being shaped by nature or nurture, most behavioural traits
are the result of an interaction between the two. Nevertheless, one or the other can
still be the dominant factor. And the study in question, to be published in June's
edition of the Journal of Child Psychology and Psychiatry, suggests that in the case of
psychopathy, the genetic side is very
important indeed.

The four researchers have drawn their
conclusion from a study of twins. The
twins in question are on the books of a    Enfants terribles
                                           Oct 21st 1999
long-term project known as the Twins
Early Development Study (TEDS), which
has been following several thousand
twins since their births in 1994 and
1995. Among other things, many of the
twins in TEDS have been assessed both      Mental Health Matters, an information resource,
for a tendency to bad behaviour            provides details about psychopathy. See also the
(“conduct disorder”, in the argot of the   Journal of Child Psychology and Psychiatry, the Twins
                                           Early Development Study, Dr Moffit, Dr Plomin, Essi
field) and for the display of what are     Viding and James Blair.
referred to as callous-unemotional traits,
such as a lack of feelings of guilt after
doing something wrong, or not having at
least one good friend. In adults, callous
and unemotional traits are symptoms of
psychopathy, and those who display such
traits in childhood frequently keep them
into adult life. The assessments were done by the children's teachers, whom years of
experience have shown are more objective and accurate than a child's parents.

As is well known, twins come in two varieties: fraternal, in which the individuals have
half their genes in common, just like ordinary siblings, and identical, in which the
individuals have all their genes in common. This means that behavioural traits with a
large genetic component are more likely to be shared by identical twins than
fraternal twins. Conversely, those traits with a large environmental component will
be shared by identical and fraternal twins in equal measure. Applying appropriate
statistical techniques to the actual amount of shared behaviour observed allows the
relative contributions of genes and environment to be worked out.

Based on the teachers' assessments, the researchers identified the naughtiest 10%
of the individuals in their sample—in other words those with severe conduct disorder.
They then subdivided these children into those with psychopathic traits and those
without and asked, in each case, whether an individual's twin showed bad behaviour,
psychopathy, or both.

Their analysis showed that bad behaviour without psychopathy has relatively little
genetic component—less than a third. By contrast, four-fifths of the difference in
behaviour between the general population and children with psychopathic traits
seems to lie in the genes.

All of this raises interesting questions. On a practical level it suggests that bad
behaviour needs to be handled differently in different children, and will be much
harder to eradicate if associated with psychopathic traits (though that does not mean
that parents and teachers should not try). On an intellectual level, it asks about the
origins of psychopathy.

Though the genes in question have yet to be identified, this result suggests they are
too abundant to be there by chance—in other words they are being kept in the
population by natural selection because psychopathic behaviour confers a selective
advantage. If it does, such an advantage probably pertains only when psychopaths
are in the minority (a state of affairs known to biologists as a balanced
polymorphism). But it does mean that far from being an aberrant behaviour,
psychopathy may be disturbingly normal.

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