Psychopathy 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 Health 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.