Nature | Editorial

The kill switch

Brain researchers and social scientists are well placed to find out what makes humans murder.

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Groups of humans have always slaughtered those who belong to other groups. The twentieth century was shot through with numerous examples, from the genocides of Armenians in Ottoman Turkey and of Jews in Nazi Europe to the massacres of ethnic rivals in civil wars in Rwanda and Bosnia during the 1990s. Today, the fundamentalist group ISIS is spooking the world with its willingness to butcher others who do not adhere to its extremist form of Islam.

Attempts to understand such events tend to focus on political reasons. But a conference in Paris last month dared to ask a different question: how, biologically speaking, do normally non-violent and psychologically stable people overcome the instinctive human aversion to killing when faced with circumstances of war or extremism? What drives them to participate in acts of genocide? This is arguably the biggest challenge for interdisciplinary dialogue across the fields that consider brain and behaviour.

All human behaviours originate in the brain, which computes cognitive and emotional information to decide what to do. So what, precisely, happens in that organ at the moment that a person’s natural abhorrence of harming others is computed out of the equation?

The organizers of last month’s conference at the Paris Institute of Advanced Studies — ‘The Brains that Pull the Triggers’ — deserve credit for even posing this question. It goes against another human instinct: to consider evil in moral rather than biological terms, as if identifying a biological signature in the brain might somehow be exploited as an excuse to absolve a person of his or her responsibility.

Neuroscientists have studied the abnormal condition of psychopathy in addition to components of normal cognition — such as the recognition of emotions in the faces of others — that may have a bearing on the problem. And psychologists and sociologists have looked at the behaviour of ordinary individuals who identify themselves with particular groups and align their behaviour with that group.

The conference brought researchers from these disciplines together, along with historians who presented sobering data on the behaviour of soldiers in wartime. One presentation included documentation from post-Second World War interrogations of hundreds of untrained German reservists who were recruited to active service in 1942 and went on to slaughter tens of thousands of Jews in Poland. Transcripts revealed that their distraught commander had allowed anyone to opt out of killing — but only 1 in 10 did so.

This is tricky terrain for academics, and many researchers at the conference admitted some discomfort at being asked to consider their findings as being relevant to the neuroscience of repetitive killings. For some of the sociologists, it felt like an attempt to medicalize a social issue. For some neuroscientists, it felt like over-extrapolation of results from much simpler experiments. In the air was an uneasy feeling that such interpretations could seem superficial and trite, and could trivialize crimes against humanity.

In fact, the researchers present made a brave contribution to what was a bold and important attempt to bring a multidisciplinary approach to one of the biggest questions facing humanity.

The answer will not come quickly, but research has already identified some useful paths to follow. Neurosurgeon Itzhak Fried from the University of California, Los Angeles, for example, proposes that ordinary people are able to become repetitive killers because changes in neural circuitry free the ideology-fed, cognitive parts of the brain from the emotional parts of the brain, which normally keep actions in check.

A better understanding of brain circuitry could not, of course, influence the political forces that create the conditions for mass murder. But discussion of such politically neutral basic neuroscience could allow progress while avoiding unhelpful rhetoric.

And findings in basic science could have a direct impact: perhaps by helping to find ways of educating people to make them less likely to succumb to ideological requests or commands to kill.

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