We are living through a particularly anxious moment in the history of American parenting. In the nation's big cities these days, the competition among affluent parents over slots in favored preschools verges on the gladiatorial. A pair of economists from the University of California recently dubbed this contest for early academic achievement the "Rug Rat Race," and each year, the race seems to be starting earlier and growing more intense.

At the root of this parental anxiety is an idea you might call the cognitive hypothesis....