In the early 1960s, a Stanford psychologist named Walter Mischel began a series of famous experiments with snacks and kids. Mischel told his subjects they could have one little treat now or two if they waited awhile. The results varied widely. As Mischel and co-author Ozlem Ayduk note in their chapter of 2004's Handbook of Self-Regulation, the definitive psychology text on willpower, the very idea of delayed gratification baffled kids under 4. But nearly 60% of 12-year-olds were able to wait the full 25 minutes until Mischel returned with the two promised sweets.
Mischel, who now teaches at Columbia, theorized that...