Cruelty often seems senseless. When five lives were taken in New York City in a random act of violence, the natural response was horror and confusion. How can the same species capable of love, empathy, and creativity also produce genocide, torture, and mass shootings?

It's tempting to think of such violence as a glitch in the system, something broken in the minds of individuals. But part of the truth, however unsettling, is that these dark impulses are not new. They're not always random. And they're not always disconnected from our evolutionary past.

Still, evolution alone doesn't explain it all. In the case of the New York City shooting, reports have pointed to chronic traumatic encephalopathy (CTE) as a likely factor. Neurological trauma, social isolation, mental illness, and easy access to weapons are all factors. Evolutionary psychology offers one lens, not the whole picture. But it's a lens that helps us confront a part of ourselves we often prefer to........

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