The clue to Breivik's cruelty lies in his brain

His lack of empathy set the stage for a deadly mix of nature, nurture and extreme ideological conviction

On July 22 last year Anders Breivik went to a summer camp on Utoya island in Norway dressed as a police officer and shot dead 69 teenagers. This week he is standing trial in Oslo and the court has to determine if he was "criminally insane" or just "criminal". If the former, he will go to a locked psychiatric ward for treatment. If the latter, he will go to prison.

Psychiatrists are divided. Last year one set of experts said he had schizophrenia, while this year new reports from different experts say he was never insane, either then or now.

Breivik denies that he has schizophrenia. It is important to his self-professed cause of stopping the "Islamification" of Europe that his actions are not explained as the disturbed behaviour of someone with no self-control or capacity for "reason". Certainly, from glimpses of Breivik on television or reading his 1,518 page manifesto 2083: A European Declaration of Independence, his behaviour does not appear to be the incoherent output of "thought disorder". Rather, his writing and his thought seems linear, carefully crafted, the work of a man with a single belief that he wishes to prove to the world in exhaustive detail and logical fashion.

Setting aside whether Breivik is sane or not, we face a bigger question: how was he capable of such cruelty? Acts of cruelty are sometimes attributed to someone being "evil" or a "monster", but these terms belong to horrific fairytales or horror movies. They do not help us understand why a man can shoot people in cold blood.

Neuroscience, however, is pointing us towards an understanding of cruelty. If Breivik were put into an MRI scanner, we could predict that the specific circuit in the brain, the empathy circuit, was undeveloped.

Empathy divides into at least two components, "cognitive" and "affective". Cognitive empathy is the drive to identify someone elseâ™s thoughts and feelings, being able to put yourself into their shoes to imagine what is in their mind. Affective empathy is the drive to respond to someone else's thoughts and feelings with an appropriate emotion.

Most of us have enough empathy to know which of our words or deeds would upset others, so we can bite our lip or sit on our hands when we sense it is prudent or kind to do so. Empathy provides the brakes on our behaviour, since without it our own selfish thoughts and wishes would burst through, unbridled, potentially bruising other people's feelings or worse. Imagine the parent who, in a fit of frustration that erodes their empathy, batters their own child.

We might liken cognitive empathy to radar, where other people's feelings are suddenly detectable. Without it, one might walk into a room where an argument has just happened with no awareness that this is the wrong time to be chatty. One might also liken cognitive empathy to musical sense: lacking it might leave the person tone deaf, singing loudly and enthusiastically in church but out of tune, unaware that other singers are inwardly cringing at the discord.

A way to understand affective empathy is to imagine sitting in the darkness in a movie. You can feel yourself identifying with a character as she says goodbye to her child for the last time and your eyes well up. We have not just decoded the situation; we have felt her feelings. Lacking affective empathy would leave you unmoved, detached. Seeing an old man stumble across the street, we not only read the situation but feel impelled to rush over and help. Lacking affective empathy would mean we could just walk by.

People with antisocial personality disorder (including psychopaths) typically have intact cognitive empathy - they have no trouble reading other people's feelings; but they have reduced affective empathy - other people's suffering is of no concern to them.

Breivik's behaviour during the trial certainly betrays his lack of affective empathy. Although he wept when he saw a video that he posted online about the evils of multiculturalism, he did not weep at the plight of his victims. His emotions seem to be self-centred, not a response to other's suffering.

His crimes - he claims his actions were part of a carefully planned project that he worked on for nine years - seem to be the result of a mind that is left free to reason, unhindered by the normal brake on behaviour that affective empathy provides. Interestingly, people with autism typically have the opposite profile to psychopaths: they have difficulties with the cognitive component (they have trouble inferring what other people might think or feel) but have intact affective empathy (it upsets them to hear of others suffering). So Breivik is unlikely to have autism.

Breivik's diagnosis must remain speculative but we know that early childhood experience has far-reaching effects on an adult's empathy levels. Breivik's parents divorced when he was a year old, his father divorced again when he was 12 and he has had no contact with his father since 1995. Certainly one route to low empathy is an absence of important parental affection in childhood, and growing up with a sense of hate. But neuroscience tells us that nature also plays a role, as there are genes that correlate with how much empathy a person has.

Nature and nurture act together to tip a person over to act in cruel ways. If we are to reduce the risks of future acts of cruelty, we must try to understand how they arise. But low affective empathy is not sufficient alone to explain Breivik's cruelty because there are people who also suffer from it who do not go on to commit murder. Low affective empathy is the precondition - it sets the stage - for cruelty, but it has to interact with other factors. In Breivik's case, his offensive ideological convictions may be one extra ingredient in the deadly mix.

Breivik is not the first person to have committed acts of cruelty motivated by ideological convictions. The young Hitler in 1923 announced he was starting a national revolution. When arrested, Hitler tried to use the trial to make political speeches, just as Breivik is trying to do. While in prison Hitler wrote Mein Kampf, which has many parallels with Breivik's manifesto. Hitler argued against the "Jewification" of Europe just as Breivik argues against its "Islamification". Both were men convinced by the rightness of their beliefs, and both were willing to sacrifice and dehumanise people to achieve their ends.

Simon Baron-Cohen is Professor of Developmental Psychology at the University of Cambridge and author of Zero Degrees of Empathy (Penguin)