Our instincts--for food, sex, or territorial protection-- evolved for life on the savannahs 10,000 years ago, not in today's world of densely populated cities, technological innovations, and pollution. We now have access to a glut of larger-than-life objects, from candy to pornography to atomic weapons--that gratify these gut instincts with often-dangerous results. Animal biologists coined the term "supernormal stimuli" to describe imitations that appeal to primitive instincts and exert a stronger pull than real things, such as soccer balls that geese prefer over eggs. Evolutionary...