Why are preschool programs becoming less effective?

Public preschool programs are heralded as an effective policy tool for promoting the development and lifelong well‐being of children from low‐income families. Recent preschool evaluations report divergent findings that are consistently weaker than those of famous demonstration programs implemented in the mid‐20th century. We provide potential explanations for these weaker effects, the most compelling of which focuses on improvements in the early childhood conditions of children not enrolling in public programs. We argue that other explanations, such as subsequent low‐quality schooling experiences, do not convincingly account for weakening program effectiveness. We do not contest whether governments should invest in effective care for young children. Rather, we focus on the current state of the evaluation evidence for programs at scale. We argue the field must take seriously the disappointing impacts of modern programs on child outcomes and strive to understand how to boost program effectiveness through rigorous, longitudinal research.