Introduction: Two-Generation Mechanisms of Child Development

Programs that aim to improve the lives of children from disadvantaged backgrounds are facing a challenge. On the one hand, scholars and policy makers agree that we must invest in children to secure our country’s future and to promote educational and economic opportunity, suggesting that we should expand programs for children, especially during early childhood.¹ On the other hand, there is a growing sense in some quarters that existing programs for children are not working as well as they could.

A few widely cited models, such as Perry Preschool and the Abecedarian Project, have demonstrated that high-quality programs can make a big difference in children’s lives.² The children who participated in these programs have shown long-term gains in educational attainment, employment, and earnings relative to their peers, and those who participated in Perry Preschool had lower rates of arrest.

The evidence from larger-scale efforts, such as Head Start and some state prekindergarten programs, is less clear-cut. On the one hand, numerous assessments of Head Start, the nation’s largest preschool program, which enrolls about 900,000 mostly disadvantaged children, have found improvements in children’s test scores, as well as their rates of high school graduation, college attendance, and delinquency, especially among children from disadvantaged backgrounds. Similarly, assessments of state prekindergarten programs, which have a much shorter history than Head Start, have found that in elementary school, the participants—especially those from disadvantaged backgrounds—had better language skills and were less likely to repeat a grade or be suspended.³

On the other hand, a recent randomized trial of Head Start found that the test score gains children experienced at the end of the program typically faded by the end of kindergarten.⁴

And a well-executed evaluation of a preschool intervention in Tennessee found a similar fade-out by the end of first grade.⁵ It’s not unusual for gains in cognitive test scores to fade—the same phenomenon occurred in the Perry Preschool and Abecedarian projects. Still, the recent Head Start and Tennessee evaluations have caused some people to doubt the efficacy of early childhood education and of universal prekindergarten more broadly.⁶

Although it’s too early to assess the long-term benefits of the new prekindergarten programs, it’s hard to be optimistic that current programs can boost poor children’s development enough to overcome the huge divide in educational achievement and economic opportunity between children from poor families and children from economically secure families. The United States has experienced a dramatic increase in income inequality over the past four decades, which, not surprisingly, has been accompanied by a growing income gap in children’s test scores.⁷ So even if the $30 billion or so that the federal and state governments spend on preschool programs and the $640 billion the nation spends on public education are having large effects, they are not large enough to compensate for the growing gap in achievement between children from high- and low-income families.⁸

The school problems of poor children stem in large part from the home environment. Numerous studies show that parents and the home environment they provide exert a continuing influence on children as they grow up.⁹ Betty Hart and Todd Risley, in their well-known study from nearly two decades ago, found major differences in the home language environments provided by poor and more affluent parents. They estimate that the average child on welfare is exposed to 62,000 words per week at home, compared with 125,000 words per week for more privileged children.¹⁰ Similarly, based on the large sample of the Panel Study of Income Dynamics, Meredith Phillips shows very large differences, all of them favoring children from more affluent families, in time spent in conversation with adults, in primary caregivers’ verbal responsiveness, and in time spent in literary activities.¹¹ The upshot is that children from poor families show up for kindergarten already far behind in school readiness, and they fall further behind during the school years.¹²

These important differences...