Men are killers. In every country, in every culture, in every age from antiquity until the present, men have committed 90 per cent of all violent crimes. It is biology, and to a lesser extent environment, argues Neil Boyd in The Beast Within, that causes men to rape, torture, maim and murder other men, women and children. Throughout the animal world, males use aggression to ensure the survival of their species and their primacy within it. It is this biological lineage, argues Boyd, that is at the root of aggression. In addition, testosterone has an indirect but strong connection to aggression. As in the animal world, what precipitates competitive acts seems to be the biological drive for sex, but what makes a man violent is his inability to deal constructively with confusion, misunderstanding and disappointment. Boyd also explores the biological and temperamental differences between men and women. Male violence, according to Boyd, is not inevitable. Although its roots are to be found in biology, the key to reducing lethal aggression lies in controlling environmental factors. It is this thoughtful, provocative and definitely controversial premise that makes The Beast Within essential reading.