Jan 31 2019

Untangling the Evidence on Preschool Effectiveness: Insights for Policymakers


Differences in how children develop are substantially linked to their learning experiences. As early as 9 months of age, the differential experiences of children growing up in low-income households and children from more affluent homes are associated, on average, with a gap in their cognitive development. The developmental gaps continue to grow all the way through preschool, elementary, and secondary school unless other learning opportunities intervene.Halle, T., Forry, N., Hair, E., Perper, K., Wandner, L., Wessel, J., & Vick, J. (2009). Disparities in early learning and development: Lessons from the Early Childhood Longitudinal Study—Birth Cohort (ECLS-B). Washington, DC: Child Trends.

Evidence from early learning programs in the 1960s and ’70s demonstrated enormous benefits for children (see Table 1). Those who attended these high-quality programs, the Abecedarian Project, Chicago Child-Parent Centers, and the Perry Preschool Project, were more ready for school and less likely to be identified as having special needs or to be held back in elementary school than children who didn’t attend. When those children grew up, they graduated high school and attended college at higher rates, and they were less likely to become teenage parents, commit crimes, or depend on welfare. Inspired by this evidence and long-term social returns on investment as high as $17 for every $1 spent,Campbell, F. A., Pungello, E. P., Burchinal, M., Kainz, K., Pan, Y., Wasik, B. H., Barbarin, O., Sparling, J. J., & Ramey, C. T. (2012). Adult outcomes as a function of an early childhood educational program: An Abecedarian Project follow-up. Developmental Psychology, 48(4), 1033–1043; Reynolds, A. J., Ou, S., & Temple, J. (2018). A multicomponent, preschool to third grade preventive intervention and educational attainment at 35 years of age. Journal of the American Medical Association Pediatrics, 172(3), 247–256; Schweinhart, L. J., Montie, J., Xiang, Z., Barnett, W. S., Belfield, C. R., & Nores, M. (2005). Lifetime effects: The High/Scope Perry Preschool Study through age 40. Monographs of the High/Scope Educational Research Foundation, No. 14. Ypsilanti, MI: High/Scope Press. many states have invested in preschool programs to provide learning opportunities that improve children’s outcomes.

A large body of research on contemporary preschool programs finds similar benefits for children’s school readiness and later outcomes. However, evaluations of two programs—Tennessee’s Voluntary Pre-K program and Head Start—found mixed results, leaving policymakers and the public confused about how to interpret the findings and what to do to ensure productive investments. 

This brief and the report on which it is based present the most rigorous available evidence on the effects of preschool and find that well-implemented preschool programs support substantial early learning gains and can have lasting impacts throughout school. We also explain how the findings from Tennessee and Head Start inform our overall conclusion that preschool is an effective intervention. We further find that the quality of a preschool program matters for its outcomes and that the method a study uses to compare children in a program to others outside the program shapes the interpretation of the research findings. When children who attend a specific preschool program are compared to those who did not attend preschool at all—as opposed to those who attended the same or another program—the benefits of preschool are clear. 

The evidence supports moving beyond the question of whether preschool “works” and focusing instead on the more pressing question of how to design and implement programs that ensure public preschool investments consistently deliver on their promise.

The Evidence

Most evaluations of preschool programs examine whether preschool effectively prepares children for school. These studies clearly show that children who attend preschool programs are better prepared for school than children who do not. Among the programs included in our review, researchers found clear benefits for participating children’s early literacy skills in 17 out of 18 where such skills were evaluated (see Figure 1). Likewise, researchers found benefits for children’s early mathematics skills in 14 out of the 16 programs where these skills were assessed. The few findings of “no difference” generally showed positive influences, though not large enough to be considered statistically significant, usually because of small sample sizes.Wong, V. C., Cook, T. D., Barnett, W. S., & Jung, K. (2008). An effectiveness-based evaluation of five state pre-kindergarten programs. Journal of Policy Analysis and Management, 27(1), 122–154.

Fewer studies examined children’s social-emotional skills and executive function at school entry by measuring outcomes such as self-control and attentiveness. Of the studies that looked at these outcomes, four out of six found benefits for at least one measure, including emotion recognition and teacher reports of student engagement and behavior. In one of the “no difference” studies, the evaluators of the program suggested that difficulty in consistently measuring these skills across different grade levels and teachers may explain the lack of significant findings.Peisner-Feinberg, E. S., Schaaf, J. M., LaForett, D. R., Hildebrandt, L. M., & Sideris, J. (2014). Effects of Georgia’s Pre-K Program on children’s school readiness skills: Findings from the 2012–2013 evaluation study. Chapel Hill, NC: University of North Carolina, FPG Child Development Institute.

The evidence examining whether the effects of preschool persist as children progress through school also paints a largely positive, though somewhat less consistent, picture (see Figure 2). Some studies found enduring effects, underscoring that long-lasting benefits are possible. Others, however, found few differences between children in a particular preschool program and children to whom they were compared in later grades.

As we describe later in this brief, there are often challenges in maintaining a comparison group over time that allows for clear interpretation of trends. Nonetheless, of the studies in our review that measure children’s literacy beyond school entry, about half found significant benefits of preschool for children’s reading performance in elementary school—in several cases persisting up to 5th grade—and the other half found little difference between the children who attended the specific preschool program and other children who remained in the comparison group throughout school.

Study methods can make a difference in results. For example, two evaluations of the same program—North Carolina Pre-K—had very different findings. One study found no effect on children’s literacy skills at the end of kindergarten,Peisner-Feinberg, E. S., Mokrova, I. L., & Anderson, T. L. (2017). Effects of participation in the North Carolina Pre-Kindergarten Program at the end of kindergarten: 2015–2016 statewide evaluation. Chapel Hill, NC: University of North Carolina, FPG Child Development Institute. and the other found benefits for children’s performance on standardized reading tests in 3rd through 5th grade.Dodge, K. A., Bai, Y., Ladd, H. F., & Muschkin, C. G. (2016). Impact of North Carolina’s early childhood programs and policies on educational outcomes in elementary school. Child Development, 88(3), 1–19. The two studies had very different designs and measured literacy skills using different tests. They also used different comparison groups. The differences in findings are likely due to these differences in research methods and timing.

Of the 13 studies that examine children’s mathematics performance throughout school, 10 document significant benefits, including some that persist well into middle school. One other study found a positive influence, though not large enough to be considered significant. Two of the studies, however, found that preschool participants performed less well than the children to whom they were compared on at least one measure of mathematics skills in the early elementary grades. These evaluations of Head Start and the Tennessee Voluntary Pre-K program are discussed in depth later in this brief. In both cases, we discuss concerns with the study design and comparison group composition in later grades. We also discuss how issues related to both program and later elementary school quality can affect the interpretation of these results.

Finally, some preschool evaluations also examine impacts on grade retention and special education placements. Among the studies that examined special education placements, most (4 out of 7) found reductions in special education placements in elementary school for participating children, and two found no effect. The other study—of Tennessee Voluntary Pre-K—found that children who participated in preschool were significantly more likely to be placed in special education when they entered elementary school.Lipsey, M. W., Farran, D. C., & Durkin, K. (2018). Effects of the Tennessee Prekindergarten Program on children’s achievement and behavior through third grade. Early Childhood Research Quarterly, 45, 155–176. In that case, involvement with the public school system at an earlier age likely led to earlier identification of underlying developmental delays.

Of the studies that measured grade retention, most (6 out of 10) found a reduction for participating children in being held back in grade. Two evaluations of Tulsa’s early childhood education programs did not find evidence of a difference between preschool participants and those in the comparison group. Both studies found fairly low rates of grade retention for all children, and in both cases, the evaluators suggested that many of the children to whom participants were compared attended other high-quality preschool programs, meaning both groups may have benefited equally from their early learning experiences. 

Lower rates of grade retention and special education placements come with significant and immediate cost savings for school systems and society. School districts spend an average of $13,119 per child each year,U.S. Department of Education, National Center for Education Statistics. (2018). The condition of education (NCES 2018-144), Public School Expenditures. a cost that is doubled whenever a student is retained in grade. Retaining a child in grade also increases the likelihood of future retentions, compounding the associated costs.Xia, C., & Glennie, E. (2005). Cost-benefit analysis of grade retention. Raleigh, NC: Duke University, Center for Child and Family Policy, 1–7. Furthermore, the annual cost of providing special education services can be more than twice that of a general education program, and early identification of special needs—and education that addresses them early on—can reduce the number of years that special services are needed, further reducing the overall costs to schools and society.

The implications of research design are clear in the case of one famous Head Start study. The study participants, who had attended Head Start, were compared to children who had either also attended Head Start, had attended another preschool program, or had attended no preschool program. Thus, the results were difficult to interpret and, in fact, showed little difference between the groups. As described in the box that follows, when Head Start participants were compared to children who did not attend any preschool program, the positive benefits of Head Start were obvious. 

Research indicates that successful programs incorporate common elements of preschool quality, such as well-qualified educators, a developmentally appropriate curriculum, and adequate learning time.Wechsler, M., Melnick, H., Maier, A., & Bishop, J. (2016). The building blocks of high-quality early education programs. Palo Alto, CA: Learning Policy Institute; NAEYC Early Learning Standards and Accreditation Criteria & Guidance for Assessment, 2017. Most or all of these elements are present in the programs that demonstrate the strongest and most persistent impacts on children.Wechsler, M., Melnick, H., Maier, A., & Bishop, J. (2016). The building blocks of high-quality early education programs. Palo Alto, CA: Learning Policy Institute; Wechsler, M., Kirp, D., Tinubu Ali, T., Gardner, M., Maier, A., Melnick, H., & Shields, P. (2016). The road to high-quality early learning: Lessons from the states. Palo Alto, CA: Learning Policy Institute.

In studies of the longer term effects of preschool programs, the importance of quality teaching in early elementary grades also should not be discounted. In addition to findings that investments in elementary schools influence the strength of ongoing preschool effects,Johnson, R. C., & Jackson, C. K. (2017). Reducing inequality through dynamic complementarity: Evidence from Head Start and public school spending (No. w23489). Cambridge, MA: National Bureau of Economic Research. researchers have found that the level of challenge provided by kindergarten teachers matters for later outcomes. A national study of kindergarten instruction found that many kindergarten teachers provide relatively uniform instruction that covers basic skills, even when alumni of a preschool program have likely already mastered these skills.Claessens, A., Engel, M., & Curran, F. C. (2014). Academic content, student learning and the persistence of preschool effects. American Educational Research Journal, 51, 403–434. It also found that too much time spent on this basic content suppresses learning gains, whereas more time spent on more advanced content is positively associated with student learning. If kindergarten does not build on what children have learned in preschool and allow them to explore new ideas, preschool attendees may become disengaged and gradually lose ground relative to their peers. 

Considerations of program quality as well as the nature of the comparison group in the Tennessee study have been raised as concerns that may account for its unexpected findings, as described in the box below. 


The weight of a sizable body of evidence indicates that preschool programs make a substantial difference in preparing children for school.Peisner-Feinberg, E. S., Mokrova, I. L., & Anderson, T. L. (2017). Effects of participation in the North Carolina Pre-Kindergarten Program at the end of kindergarten: 2015–2016 statewide evaluation. Chapel Hill, NC: University of North Carolina, FPG Child Development Institute. The evidence about continued effects beyond school entry is also positive, but less consistent. Sorting out these findings requires an examination of the way that different studies construct comparison groups—whether children in those groups are truly comparable to the children who attended the preschool program under study and whether they themselves attended a different preschool.

In order to generate meaningful impacts, early learning experiences need to be rich and engaging.Wechsler, M., Melnick, H., Maier, A., & Bishop, J. (2016). The building blocks of high-quality early education programs. Palo Alto, CA: Learning Policy Institute. Implementing a high-quality preschool program well—offering compensation and support that attract and retain a highly qualified workforce; a program day that provides adequate, productive learning time and activities; and child assessments used to individualize learning—is complex and often expensive.Meloy, B., Gardner, M., Wechsler, M., & Kirp, D. (2019). “What Can We Learn From State-of-the-Art Early Childhood Education Programs?” in Reynolds, A. R., & Temple, J.A. (Eds.). Sustaining Early Childhood Learning Gains: Program, School, and Family Influences, pp. 101–132. Cambridge: Cambridge University Press. Finally, although preschool quality is important, even the highest quality preschool cannot inoculate children from the detrimental effects of poverty or poor elementary and secondary schools. Sustained benefits likely require investments in children and their families that are also sustained from preschool through grade school and beyond. 

Untangling the Evidence on Preschool Effectiveness: Insights for Policymakers by Beth Meloy, Madelyn Gardner, and Linda Darling-Hammond is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

Funding for this brief and the full report on which it is based was provided by the Heising-Simons Foundation, along with the general operating support from the Ford Foundation, the William and Flora Hewlett Foundation, and the Sandler Foundation.

Our Review

We reviewed studies that used strong research designs (randomized experiments or those with well-controlled comparison groups) to understand the impacts of 21 public preschool programs at school entry and beyond. For the studies of the impact of preschool on children’s school readiness, which has been extensively researched, we were extremely selective—including only evaluations with the strongest research designs (experiments and strong quasi-experiments). There are far fewer studies that follow preschool participants into the early elementary grades and beyond. For this timeframe, we included a wider range of research designs but maintained a high bar for the strength of each evaluation. Table 2 at the bottom of this page lists the evaluations included in our review.

Note: See the full report for a list of sources and a discussion of the methodology.