Does preschool improve school preparation and performance? Researchers at the National Bureau of Economic Research report "prekindergarten increases reading and mathematics skills at school entry, but also increases behavioral problems and reduces self-control. Furthermore, the effects of prekindergarten on skills largely dissipate by the spring of first grade, although the behavioral effects do not." These findings add to a large body of evidence reported in Assessing Proposals for Preschool and Kindergarten showing preschool is not unequivocally beneficial for all children on all measures. In fact, sometimes preschool is harmful to children on certain outcome measures. Preschool programs are neither necessary nor sufficient for academic achievement.