A review of data from 67 high-quality interventions — all of which included some degree of pre-literacy and early math skill-building and most of which targeted economically disadvantaged children — found that the effects preschool faded startlingly fast: falling by half within a year and by half again two years later.

The Washing Post article.

Paper by Drew Bailey, Greg J. Duncan, Candice L. Odgers & Winnie Yu: Persistence and Fadeout in the Impacts of Child and Adolescent Interventions