Psychopaths are born, not made

Economist:

The four researchers have drawn their conclusion from a study of twins. The twins in question are on the books of a 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 for a tendency to bad behaviour (“conduct disorder”, in the argot of the field) and for the display of what are 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.

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