A judge’s decision to reduce a killer’s sentence because he has genetic mutations linked to violence raises a thorny question – can your genes ever absolve you of responsibility for a particular act?

In 2007, Abdelmalek Bayout admitted to stabbing and killing a man and received a sentenced of 9 years and 2 months. Last week, Nature reported that Pier Valerio Reinotti, an appeal court judge in Trieste, Italy, cut Bayout’s sentence by a year after finding out he has gene variants linked to aggression. Leaving aside the question of whether this link is well enough understood to justify Reinotti’s decision, should genes ever be considered a legitimate defence?

No, says Nita Farahany, a legal scholar at Vanderbilt University in Nashville, Tennessee, who tracks the use of behavioural genetics in the courtroom. She says genes may provide a guide as to how someone is likely to behave, but they will never tell us why they committed a specific act. “It doesn’t tell us why they did the thing they did and that’s what criminal cases are ultimately interested in.”

Double-edge

What’s more, the gene argument seems to cut both ways. Reinotti viewed Bayout’s genes as mitigating his crime, but Farahany has noticed that US courts are increasingly using genes in evidence for the prosecution. “It’s just as likely to be used against a criminal defendant as for,” she says. “People don’t recognise the double-edged potential of this evidence.”


Even if technological advances allow researchers to better explain how genes and environment influence violent behaviour, courts may not take notice, says Terrie Moffitt, a geneticist at King’s College London and Duke University in Durham, North Carolina, whose previous work influenced the Italian court’s decision.

Links between inherited genes, environment and violence are already bolstered by family histories and twin studies, she says. “Everything we know about family history still doesn’t diminish our own responsibility for how we make choices.”