New Scientist has a fantastic article on making sense of cognitive genetics studies, the science that links certain versions of genes to behaviour, by taking the use and abuse of the MAOA gene as an example. If the name doesn’t ring a bell you may remember it being dubbed ‘warrior gene’, which as well as being inaccurate, was one of its least embarrassing moments.
For many decades, genetics and psychology only really interacted with the twin study, which, by comparing the differences between identical and non-identical twins, can indicate how much of the difference in the twins you’ve studied is due to the environment and how much has been inherited.
As it became possible to identify individual genes, and more importantly, as automated ‘gene chip‘ technology made this economical, studies began looking at differences between groups of people distinguished by simply having different versions of the same gene.
The idea is to see how a single gene influences behaviour, but because the gene and the everyday effect are so distant (it’s like trying to detect the effect of a day of farm weather on the flavour of your lunch) the story often gets mangled in the retelling.
The New Scientist article, by Not Exactly Rocket Science’s Ed Yong, tells the story of MAOA and its headline-making link with aggression, but it also serves as an essential hitchhiker’s guide to the science and pitfalls of linking genetics with behaviour.
However, the clearest sign yet that the gene is no ruthless determinant of behaviour came in 2002 when Avshalom Caspi and Terrie Moffitt of Duke University in Durham, North Carolina, published their findings about a sample of 442 men from New Zealand who they had followed from birth. A third of these men carried the MAOA-L variant. Now, aged 26, this group was indeed more likely than the others to have developed antisocial disorders and violent behaviour – but only if they had been poorly treated or abused as children. Moffitt and Caspi concluded that the so-called “warrior gene” affects a child’s sensitivity to stress and trauma at an early age. Childhood trauma “activates” bad behaviour, but in a caring environment its effect is quashed.
Since then, similar interactions between nature and nurture have become part and parcel of the MAOA story. Carriers of MAOA-L are more likely to show delinquent behaviour if they were physically disciplined as children. They are also more likely to be hyperactive in late childhood if their first three years were stressful, and to develop conduct disorders if their mothers smoked cigarettes while pregnant with them. The list goes on. Likewise, Beaver found that MAOA-H carriers were more likely to commit fraud, but only if they hung around with delinquent peers.
Link to NewSci article on MAOA, genes and behaviour.