On May 11, 2005, a jury convicted Pete Terrazas of murdering his next-door neighbor, Miguel Ruiz. Terrazas had been dating Ruiz's housekeeper, Maria Santillana, whom he deeply loved. When she abruptly broke off the relationship, Terrazas concluded that she had begun an affair with Ruiz. Terrazas loaded his .410-gauge shotgun, went over to his neighbor's driveway, blasted Ruiz in the back and then took deadly aim at the man's chest. Pete Terrazas had never before been violent. Nor had Scott Peterson before he killed his wife, Laci. Nor had Clara Harris before she ran over her adulterous husband with her Mercedes in a hotel parking lot in Houston.
My own interest in studying murder began when I witnessed a close friend, a highly accomplished academic, fly into a murderous rage and come frighteningly close to killing his wife. This raised a disturbing question: Could "normal" people become killers? In seven years of research on murderers, I discovered that an astonishing 91% of men and 84% of women in five different cultures have had at least one vivid fantasy of committing murder.
As my research continued, I became convinced that we all have the capacity to become murderers. There's a compelling reason why. Over the long sweep of deep time, killing has conferred such powerful advantages in the ruthless game of reproductive competition that natural selection has forged in all of us minds designed to murder. Murderer's genes prevailed over those of their unfortunate victims, and we are their descendants.
Our minds are designed to kill. It's part of human nature.
Previous theories about why people kill typically invoke single factors -- the murderer is pathological, or the violent product of poverty, or warped by child abuse, poor parenting or exposure to media violence. But I concluded that every one of these theories is wrong.
The unfortunate fact is that killing has proved to be a disturbingly effective solution to an array of adaptive problems in the unforgiving evolutionary games of survival and reproductive competition: preventing injury, rape or death; protecting one's children; eliminating a crucial antagonist; acquiring a rival's resources; securing sexual access to a competitor's mate; preventing an interloper from appropriating one's own mate; and many others. The logic of evolutionary struggle is all about reproductive competition. Those strategies that lead to greater reproductive success are selected for, over eons of evolution, and come to characterize our species.
The statistics on contemporary circumstances in which people kill reveal precisely what's at stake, reproductively speaking -- rivals who have poached on one's mate, romantic partners who have defected to an interloper, women who are pregnant with another man's child, and children whose lives are in danger when they live with genetically unrelated stepparents.
Evolutionary theory also explains why men kill so much more than women -- 87% of killers worldwide are men. Women are the more valuable reproductive resource because of a fact of human reproductive biology: Women, not men, bear the burdens of the nine-month investment to produce a child. Competition is always fiercest among the sex that invests less. As a result, men battle to avoid mating failure and to "win big" by getting to the top to mate with desirable (and sometimes multiple) women. Mating is inextricably intertwined with murder.
If we all have mental mechanisms designed for murder, why don't more of us kill? For one thing, killing is so costly for victims that natural selection has fashioned finely honed defenses -- anti-homicide strategies -- designed to damage those who attempt to destroy us. We kill to prevent being killed, so attempting murder is a dangerous strategy indeed. Second, we live in a modern world of laws, judges, juries and jails, which have been extremely effective in raising the cost of killing. Homicide rates among traditional cultures lacking written laws and professional police forces are far higher than those in modern Western cultures. Among the Yanomamo of Venezuela and the Gebusi of Africa, for example, more than 30% of men die by being murdered.
It may be disturbing to think of killing as evolutionarily adaptive and part of human nature, but this does not mean approval or acceptance of murder. I would suggest instead that those who create myths of a peaceful human past, who blame killing on the contemporary ills of modern culture and who cling to single-variable theories that have long outlived their scientific warrant are the ones who tread on dangerous moral ground. The problem of murder cannot be solved by wishing away undesirable aspects of human nature.