The left-leaning Brookings Institution on universal preschool:

This relatively recent explosion of public pre-K programs has been underpinned by research findings from two iconic preschool interventions from 30 to 40 years ago whose participants have been followed into adulthood, the Perry Preschool Project and the Abecedarian Project... In my view, generalizations to state pre-K programs from research findings on Perry and Abecedarian are prodigious leaps of faith. Perry and Abecedarian were multi-year intensive interventions whereas state pre-K programs are overwhelmingly one year programs for four-year-olds. Costs per participant for Perry and Abecedarian were multiples of the levels of investment in present-day state preschool programs, e.g., $90,000 per child for Abecedarian.[3] Both Perry and Abecedarian were small hothouse programs (less than 100 participants) run by very experienced, committed teams, whereas widely deployed present day preschool programs are, well, widely deployed. The circumstances of the very poor families of the Black children who were served by these model programs 30 to 40 years ago are very different from those faced by the families that are presently served by publicly funded preschool programs.

But we do have the Head Start findings I reviewed last week and we should not ignore them in thinking about state pre-K. Head Start spends about twice as much per child per year as states ($8K per child per year for Head Start vs. $4K for state pre-K). And Head Start includes many program components that are advocated by early childhood experts such as health, nutrition, and parental involvement that are much less prevalent in state pre-K. If a year of Head Start does not improve achievement in elementary school, should we assume that a year of state pre-K does?

These three studies fall far short of providing a convincing case for investment in universal pre-K: The Georgia study finds impacts that are at best very small and do not pass a cost-benefit test. The Texas study provides evidence for value in a targeted program and is silent on the effectiveness of a universal program. The Tulsa study and other studies that use a design that compares children who just meet or just miss the age cut-off for pre-K can't estimate the impact of state pre-K because they are comparing children that may differ in many experiences in addition to their participation in state pre-K.