As I read through their responses back in my office, I became mesmerized. Nothing had prepared me for the outpouring of murderous thoughts my students reported. These were intelligent, well-scrubbed, mostly middle-class kids, not the gang members or troubled runaways one might expect to express violent rage, yet most of them had experienced at least one intense episode in which they had fantasized about killing someone. As I sat in my office analyzing these homicidal fantasies, I realized that carried-out kills were just the tips of the deep psychological iceberg. Could actual murder be only the most flagrant outcome of a fundamental human drive to kill? I wondered. Do our minds really course with homicidal thoughts?

Pursuing this line of research, my lab went on to conduct the largest scientific study ever carried out on why people have homicidal fantasies, and the specific circumstances in which they contemplate killing. This groundbreaking international study involved more than four thousand individuals from San Antonio to Singapore, who were interviewed intensively. According to our findings, 91 percent of men and 84 percent of women have had at least one vivid fantasy about killing someone. The answer appeared to be, yes, our minds do pulse with thoughts of murder.

As I contemplated this finding, with the knowledge that the human mind has been exquisitely finely tuned by evolution, I began to suspect that these fantasies were the expressions of deep psychological mechanisms—of evolutionary programming—predisposing us to kill. Six years of near-obsessive subsequent research has led me to the conclusion that yes, the human mind is indeed hard-wired for killing, and that all the many kinds of murder—from crimes of passion to the methodically planned contract kill—follow the same deeply ingrained impulses. There is a fundamental logic to murder, which is ruthless but rational.