This is a guest post from Vanessa Woods, author of the new book, Bonobo Handshake. Vanessa is a Research Scientist in Evolutionary Anthropology at Duke University and studies the cognition of chimpanzees and bonobos in Congo. We like to think that murderers are psychopaths, with some kind of abnormal psychology that would never appear in us, or someone we know. And yet most of us think we would kill in certain situations, like if we were at war, or someone was about to kill a person we loved. How 'natural' is this instinct in us, and can we ever obliterate it completely? In my new book, Bonobo Handshake, I talk about lethal aggression in one of our closest relatives, chimpanzees. Chimpanzees and humans have a lot in common when it comes to killing: #1 Killers are mostly male. Though female chimpanzees can participate in killing, usually the killers are males. In humans too, the FBI reported in 2005 that 89% of killers are male . #2 Males usually attack when the ratio is 3:1. Wrangham and Wilson reported that both chimpanzees and young men in gangs attack when they outnumber their victim 3 to 1 or more. The reason for this? This is the minimum number ...