Unlike established K-12 public schooling, preschool programs have had to justify their claim for public funding by demonstrating their effectiveness. In the 1960s, longitudinal studies of the Early Training Program,1 the HighScope Perry Preschool Program,2 and others showed improvements in intellectual performance that promised to be permanent, but then disappeared a few years later, leading to the perception that the effects of preschool programs were transient. In the 1980s, the Consortium for Longitudinal Studies3,4 reported positive long-term outcomes related to children’s grade placement and mental disabilities 5 years and more after preschool. The HighScope Perry study,5 the Abecedarian study,6 and the Chicago Child-Parent Centers study7 subsequently found even longer-term benefits into adulthood in terms of school achievement, high school graduation, adult earnings and employment, crime prevention, and economic return on investment. Some more recent studies have also found program effects,8-10 but others, notably the Head Start Impact Study,11 demonstrated that the known benefits of high-quality preschool programs do not generalize to all preschool programs.