Greg Duncan and Katherine Magnuson:

Programs beginning before 1980 produced significantly larger effect sizes (.33 standard deviations) than those that began later (.16 standard deviations). Declining effect sizes over time are disappointing, as we might hope that lessons from prior evaluations and advances in the science of child development would have led to an increase in program effects over time. However, the likely reason for the decline is that counterfactual conditions for children in the control groups in these studies have improved substantially. We have already seen in Figure 1 how much more likely low-income children are to be attending some form of center-based care now relative to 40 years ago. This matters because, though center-based care programs have varying degrees of educational focus, most research suggests that center-based care is associated with better cognitive and achievement outcomes for preschool age children (NICHD Early Childcare Research Network and Duncan 2003).
Even more impressive are gains in the likely quality of the home environment provided by low-income mothers, as indexed by their completed schooling. In 1970, some 71 percent of preschool age children in the bottom 20 percent of the income distribution had mothers who lacked a high school degree, while only 5 percent of the mothers had attended at least some postsecondary schooling…