Scientists have used this technology to scan the active brains of psychopaths and murderers, and compare them to people with ‘normal’ brain activity. One striking result: in psychopaths’ brains, the prefrontal cortex, the seat of executive function, fails to adequately regulate signals coming from the more primitive and impulse-driven part, called the amygdala. The tentative conclusion is that while psychopaths might know they’re about to do something wrong, they lack the neural circuitry to resist the impulse.

Genetics research is most controversial of all. A foundational study began in 1978, when a woman with extremely violent men in her family approached Han Brunner, a clinical geneticist at the University of Nijmegen in Holland, and asked if he could do some genetic testing. Brunner agreed, and undertook a 15-year study in which he tracked down several generations of the men, categorised their behaviours, and took blood samples. He found they all had a defect in a particular gene. That gene encodes for MAOA (monoamine oxidase), an enzyme that breaks down neurotransmitters associated with agitation and alertness. Brunner theorised that the absence of that gene could lead to a build-up of the neurotransmitters, setting up a biochemical hair-trigger.

Other scientists tested the idea by breeding laboratory mice without the MAOA gene. The gene-deficient rodents attacked their cage-mates much more readily than untampered-with mice. Later, as part of their massive Dunedin study, Caspi and Moffitt tracked the behaviour and took saliva samples every few years from more than 400 young men as they grew from age three to 26. After tabulating their results, in 2002 they reported that low levels or absence of the MAOA gene, coupled with a history of child abuse, increased the risk that a person would grow up to be violent. In other words, those children with the genetic abnormality were less able to shake off the abuse they suffered and become normal, non-violent adults.