What makes peaceful neighbors become mass murderers

On which data did you base your hypothetical Syndrome E?
Historians had analysed personal accounts from numerous massacres, such as of Armenians in 1915, the European Jews in the Second World War, Cambodians during the Pol Pot regime and the ethnic killings in Bosnia and Rwanda in the 1990s. I also gathered information from the few social-science experiments available at the time, such as the famous Milgram experiment [which studied the willingness of someone to inflict on another when obeying authority figures]. I was particularly struck by Christopher Browning’s 1992 book Ordinary Men, which described the testimony of hundreds of middle-aged, non-politicized German reservists who were taken to Poland in 1942. In a short time, most became efficient killers, participating in the shooting of 38,000 Jews who had been rounded up by the Nazis, and herding 45,000 more into trains destined for the gas chambers. Their commander allowed the reservists to opt out, but only 10% or so decided not to kill. I felt that the transformation into repetitive killer had to have a biology behind it — all of our behaviour is guided by brain activity.

What are the main features of the syndrome?
There was a myth that the primitive brain is held in check by our more-recently evolved prefrontal cortex, which is involved in complex analysis, and that the primitive, subcortical part takes over when we carry out brutal crimes such as repetitive murder. But I saw it the other way around. The signs and symptoms that I gathered in my research indicated that the prefrontal cortex, not the primitive brain, was responsible, because it was no longer heeding the normal controls from subcortical areas. I called it ‘cognitive fracture’ — the normal gut aversions to harming others, the emotional abhorrence of such acts, were disconnected from a hyper-aroused prefrontal cortex. I also proposed a neural circuitry in the brain that could perhaps account for this. In brief, specific parts of the prefrontal cortex become hyperactive and dampen the activity of the amygdala, which regulates emotion.