The photographs from Iraq's Abu Ghraib prison have forced us once again to look horrible human nature in the eye, just as the photographs of emaciated prisoners stacked up like cordwood at the Bergen-Belsen death camp did in 1945. How could these things happen? How could civilized people act so? How could American soldiers do these things and then give the camera a big grin and a thumbs up? Indeed, how could a Christian do such things? (One of the perpetrators of the Abu Ghraib atrocities has identified himself as a Christian.)

We all tend to distance ourselves from such evidence of human evil. In 1997 theologian Jean Bethke Elshtain wrote in Books & Culture about historians who foolishly try to locate the reasons for the Holocaust in the character of the German people. But Christian scholars, she said, "must, or should, introduce the problem of human evil … and attempt to show the ways in which systematic and well-organized murder is, alas, an immanent possibility in human affairs given the right set of circumstances."

Social science experiments help us face the biblical truth about human depravity. In the summer of 1971, Stanford psychologist Philip Zimbardo conducted a now-famous prison simulation experiment. Volunteers were recruited and randomly assigned to be "guards" and "prisoners."

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Zimbardo uses this experiment and many other studies of human evil to argue against what he calls "dispositionalism." Dispositionalism locates a propensity to certain evils in particular individuals and not others.

But Zimbardo marshals evidence showing that certain situations unleash the propensity for evil in ordinary individuals.

From 1985 to 1992, when Youth Direct president Don Smarto taught criminal ...