I am not a psychiatrist, psychologist or neurobiologist, but to a lay reader there seem to be limitations in describing pathological behavior in terms of zero degrees of empathy. He cites the example of a Nazi guard who forced a boy to put a noose around his friend’s neck as “cruelty for its own sake.” But rather than zero degrees of empathy, it seems instead that the guard possessed six degrees of anti-empathy: The guard acted in the cruelest way he could think of, fully understanding how devastating the act would be to both boys.

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“What leads an individual’s ‘Empathizing Mechanism’ to be set at different levels?’ ”Dr. Baron-Cohen asks. “The most immediate answer is that it depends on the functioning of a special circuit in the brain, the empathy circuit,” which he maps in great detail. (This question — and many sentences in the book — could have used an editor. Does he really mean that an individual’s empathy mechanism goes up and down? I don’t think so. I think what he’s trying to say — and in fact does say at a later point — is “What determines where an individual falls on the spectrum?”)

Further, what causes the same neurological circuitry to produce behaviors as different as Zero N, Zero P and Zero B? The dual answer is environment (extreme emotional deprivation or social pressure, as in Nazi Germany) and genes. The discussion of empathy genes is crammed with caveats and warnings: “I hope this book will not be misunderstood as arguing that empathy is wholly genetic”; “I have put quotation marks around genes for empathy”; “we examine evidence that some genes are associated with...” (italics his). The editors dispense with all this hedging and call the chapter “The Empathy Gene.” (The American publishers did the same with his caveats about the word “evil,” using it in the title. The British edition was called “Zero Degrees of Empathy.”)

In his final chapter, “Reflections on Human Cruelty,” Dr. Baron-Cohen addresses perhaps the most central question: If zero degrees of empathy is a “form of neurological disability, to what extent can such an individual who commits a crime be held responsible for what they have done?”

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Does this hypothesis mean that there is no such thing as individual responsibility, free will? Possibly. But sensibly, Dr. Baron-Cohen finds that prison is necessary for the most serious crimes, for three reasons: to protect society, to signal disapproval and to restore some sense of justice to the victim or the victim’s family. (He does not believe in capital punishment.) For lesser crimes, though, imprisonment may not be the answer.

Finally, zero empathy is not necessarily negative. In what he acknowledges is a controversial idea, he maintains “there is at least one way in which zero degrees can be positive.” Preparing for the howl of dissent, he adds: “It seems unthinkable, but bear with me.” People with Asperger’s syndrome also fall on the zero end of the scale, but they are Zero Positive. Zero Positive is almost always accompanied by high scores on the systemizing scale (and can lead to genius). In addition, the way “their brain processes information paradoxically leads them to be supermoral rather than immoral.”

My guess is that even suggesting these two conditions are related in some way will be inflammatory, though in the context of the book the discussion seems reasonable, and in no way does Dr. Baron-Cohen equate the two — except that they have in common zero degrees of empathy.

At the core of this deceptively simple book is the question of the nature of cruelty. In the last and most philosophical chapter Dr. Baron-Cohen discusses situations in which an individual who is not otherwise lacking in empathy may behave cruelly. Citing the philosopher Hannah Arendt’s term “the banality of evil,” and discussing the work of Stanley Milgram and Philip Zimbardo in which ordinary people exhibited cruel behavior, he acknowledges that in most of us empathy may be suspended temporarily, under certain circumstances.

This is a frightening thought, but one borne out not only by research but by history. Dr. Baron-Cohen’s hypothesis that cruelty is merely the zero end of a continuum on which we all fall makes that possibility more comprehensible.