Risk-taking behaviors have been the subject of much speculation, from Sigmund Freud's belief that dare-devil stunts arise out of humans' innate "death drive," to some modern psychologists' view that dangerous activities can make us feel more alive. In general, we think of risky behavior as encompassing activities only a handful of courageous, or "crazy," people would attempt, including skydiving, rock climbing, cliff jumping, or other dramatic exploits. In reality, though, risk-taking behaviors also include more mundane acts, like having unprotected sex, gambling, robbing banks, and taking drugs. The reasons for these behaviors are complex, although not mysterious, and can mean different things to different people. In general, though, as poet Robert Browning wrote, "Our interest's on the dangerous edge of things."

One commonly accepted theory about why people do risky things has to do with evolution. This theory holds that in one of the deep...