Dr Mairi Levitt looks at how lawyers are increasingly looking to use defendants’ genes to defend criminal behaviour, despite the evidence for its use being largely unconvincing

Reducing a convicted murderer’s sentence because they have certain genes sounds a little far-fetched, but it has already happened. In Italy and the US, a common genetic variant, present in around a third of Caucasian men, has been successfully used in the defence of violent criminals. Yet both the science and the ethics of the issue are still very much under debate.

The variant in question is an allele conveying low activity of monoamine oxidase A (MAOA)[1], an enzyme involved in regulating the metabolism of several neurotransmitters, including dopamine and serotonin, which influence brain function (Shih et al.,1999).

The first attempt to use evidence of MAOA levels as a mitigating factor for a convicted murderer was in the US in 1994. Research published the previous year had found no MAOA function in five male members of a Dutch family who exhibited low IQ and abnormal violent behaviour (Brunner et al., 1993). The MAOA gene is X-linked, and these men and others in successive generations all exhibited problem behaviour including impulsive aggression, arson and rape.