From Gaza to the streets of Northern Ireland, from the massacres of hundreds of thousands in Rwanda to the casually brutal killing of Baby P in Haringey, our newspapers and TV screens are daily filled with the violent images of humanity's inhumanity. Religious leaders speak of sin, of human wickedness. Politicians respond with tired routine phrases: "evil", "cowardly". But why does such cruelty exist? Are we all capable of such atrocities? Could I be, am I, also cruel? Sadly, the answer, according to Kathleen Taylor's wise and timely book, is yes. In the psychology laboratory, students, at least in the US, can be persuaded to act like brutal prison guards prepared to inflict intolerable pain on their victims. It is not psychopathic monsters but "ordinary men" (and women) in Nazi Germany, in Bosnia, in Vietnam, in Iraq, who are capable of acts the mere recital of which is enough to turn the stomach and make one wish to close one's eyes and stop one's ears.
Trained in philosophy and neuroscience, Taylor seeks to describe and define cruelty, to distinguish between callous brutality and sadism, and ground them in the workings of the human brain and evolutionary theory. Thus only humans can be cruel, because cruelty is a concept that has meaning only in the context of morality, and morality is an evolved property possessed only by humans (and rudimentarily by our closest evolutionary neighbours). We may perceive cats playing with the mice they hunt as cruel, but this is a human, not a cattish, response. Morality evolved because it is a necessary survival strategy for social animals, whose infants require years of care before they are capable of independent living. But what is perceived as cruel is dependent on context and culture. Why, for instance, is targeted assassination by dropping a one-ton bomb from a plane on to the house of a suspected enemy not cruel, whereas a suicide bomber who immolates himself as well as his victims is?
The first step in Taylor's argument is to extract these acts of violence from their specific social, cultural and political contexts and to seek a common underlying mechanism. She locates this in "otherisation", a universal way of thinking that separates "us" from "them", and enables "us" to treat "them" as Untermenschen. There's a certain lack of self-inspection at this point in the book. Otherisation is a term derived from cultural studies, and particularly from Edward Said's writings, but in her quest to locate the process in human genetics, developmental and evolutionary history, Taylor seems unaware of the ways in which her own universalising "we" turns out to be a well-educated, bien-pensant, white, British post-Christian, content in her writing to otherise Jews, Muslims, Catholics ... Abstracting herself from her own analysis does impoverish it.
For Taylor, otherisation is a process grounded in basic human emotions, our bias towards pleasure and avoidance of pain. Perceiving others as "others" causes fear, anger or disgust, universal "primitive" responses to threats whose physiological mechanisms are relatively well understood. These emotions evolved to enable our forebears to escape predators, fight enemies or reject polluted food. "We" therefore respond to others in ways appropriate to these hard-wired responses. Thus Nazis regarded Jews and Gypsies as disease vectors, viruses contaminating Aryan society, which needed to be cleansed from such contamination - a metaphor that is repeated by perpetrators of genocide as widely apart as Cambodia and Rwanda. Threatening ideas are also like slow-acting pathogens or poisons, and evoke disgust responses requiring the contaminated carriers of such ideas to be avoided or eliminated - Taylor notes wryly the way in which Richard Dawkins refers to religion in this way as a "virus".
She is cautious about reducing complex social behaviours to brain processes, but she is not always successful. Thus she suggests that "we" behave to others in fear, anger or disgust because of our firmly held beliefs about them. Beliefs, she goes on, are expectations. "I believe the sun will rise tomorrow" is equivalent to "I expect the sun will rise tomorrow", and expectations are coded by experience into patterns of neural connectivity. But "I believe in the truth of the Bible/Qur'an" is not an expectation in the same sense as my belief that the sun will rise. And there is no evidence of which I am aware to support her claim that the stronger the belief, the stronger the connections between ensembles of neurons in the brain. A "strong" belief is not strong in the same way that a muscle is strong.
Otherisation may explain why "we" treat others as threatening, but it still doesn't explain why we are callously brutal or even sadistic. Efficiently killing one's enemies makes sense, but why play cat and mouse with them? Torturing a victim to extract secret information may be rationalised, but if there is no obvious gain why cause gratuitous pain? Although she wisely distances herself from the wilder excesses of evolutionary psychology, Taylor's framework is strictly Darwinian, and such actions seem inefficient in the context of natural selection. Rape is common enough in war, but to murder the raped woman subsequently doesn't fit the ultra-Darwinian prediction that men seek to spread their genes into the next generation. To explain male brutality verging on sadism, Taylor invokes sexual selection - that women might prefer demonstrably brutal men because they carry desirable genes. Yet this scarcely matches the evidence of such men's brutality towards women, often their own partners - hardly a trait that could have been sexually selected.
Cruelty is a brave attempt to come to terms with one of the foulest of human traits. Taylor is surely right that any explanation will need to include biological as well as social accounts. And it will also need to explain why most of us for most of the time are not cruel, and do not abuse or murder our lovers, children, passers-by in the street, or those with a different skin colour or religion. There are clearly many ways of explaining why we behave as we do in any particular context. As extraordinary people are cruel in ordinary times, and ordinary people cruel in extraordinary times, biological and psychological accounts are likely to be of most help in the first, and social, political and economic ones in the second case. Those in search of a single unifying theory are likely to be disappointed.
• Steven Rose's The 21st Century Brain: Explaining, Mending and Manipulating the Mind is published by Vintage.