For a readable overview of what is actually known, rather than hoped, a useful starting point is "The Current State of Scientific Knowledgeon Pre-Kindergarten Effects" what was put together by a "Pre-Kindergarten Task Force of interdisciplinary scientists" convened by the Brookings Institution and the Duke University Center for Child and Family Policy. The participants are: "Deborah A. Phillips of Georgetown University, Mark W. Lipsey of Vanderbilt University, Kenneth A. Dodge of Duke University, Ron Haskins of the Brookings Institution, Daphna Bassok of the University of Virginia, Margaret R. Burchinal of the University of North Carolina at Chapel Hill, Greg J. Duncan of the University of California-Irvine, Mark Dynarski of the Brookings Institution, Katherine A. Magnuson of the University of Wisconsin-Madison, and Christina Weiland of the University of Michigan "
A short summary of the findings would be that a number of pre-K programs have a short-term effect in helping children be more ready for kindergarten, especially for children from disadvantaged backgrounds. But not all programs show such effects in the short-term, and whether the effects continue or fade after a few years of schooling is quite unclear. Here is their consensus statement:
"The Task Force reached consensus on the following findings, conclusions, and recommendation:
"Studies of different groups of preschoolers often find greater improvement in learning at the end of the pre-k year for economically disadvantaged children and dual language learners than for more advantaged and English-proficient children.
"Pre-k programs are not all equally effective. Several effectiveness factors may be at work in the most successful programs. One such factor supporting early learning is a well implemented, evidence-based curriculum. Coaching for teachers, as well as efforts to promote orderly but active classrooms, may also be helpful.
"Children’s early learning trajectories depend on the quality of their learning experiences not only before and during their pre-k year, but also following the pre-k year. Classroom experiences early in elementary school can serve as charging stations for sustaining and amplifying pre-k learning gains. One good bet for powering up later learning is elementary school classrooms that provide individualization and differentiation in instructional content and strategies.
"Convincing evidence shows that children attending a diverse array of state and school district pre-k programs are more ready for school at the end of their pre-k year than children who do not attend pre-k. Improvements in academic areas such as literacy and numeracy are most common; the smaller number of studies of social-emotional and self-regulatory development generally show more modest improvements in those areas.
"Convincing evidence on the longer-term impacts of scaled-up pre-k programs on academic outcomes and school progress is sparse, precluding broad conclusions. The evidence that does exist often shows that pre-k-induced improvements in learning are detectable during elementary school, but studies also reveal null or negative longer-term impacts for some programs.
"States have displayed considerable ingenuity in designing and implementing their pre-k programs. Ongoing innovation and evaluation are needed during and after pre-k to ensure continued improvement in creating and sustaining children’s learning gains. Research-practice partnerships are a promising way of achieving this goal. These kinds of efforts are needed to generate more complete and reliable evidence on effectiveness factors in pre-k and elementary school that generate long-run impacts.
"In conclusion, the scientific rationale, the uniformly positive evidence of impact on kindergarten readiness, and the nascent body of ongoing inquiry about long-term impacts lead us to conclude that continued implementation of scaled-up pre-k programs is in order as long as the implementation is accompanied by rigorous evaluation of impact."I suppose it is almost inevitable that a group of academics end up advocating additional research. The volume follows this consensus statement with a number of short and readable individual essays on various aspects of pre-K education. Here, I'll add a few thoughts of my own.
1) This summary of the state of evidence about pre-K programs is quite mainstream. Indeed, I've previously posted here and here about summaries that reached similar conclusions.
2) Some of the highest estimates of returns to pre-K education are probably not generalizable to a broader program. For example, in a short chapter on "The Costs and Benefits of Scaled-Up
Pre-Kindergarten Programs" in this volume, Lynn A. Karoly writes (footnotes omitted):
"Estimates of the high returns from investing in high-quality pre-k programs largely rest on two pre-k program impact evaluations: the Perry Preschool program where the returns based on the age-40 follow-up are estimated to be as high as 17-to-1, and the Chicago Child-Parent Centers (CPC) program, where the impact estimates as of the age-26 followup indicate returns of close to 11-to-1. The Perry Preschool program, while well known, is also acknowledged to be a small-scale demonstration program, implemented in the 1960s with exceptionally high standards and serving a highly disadvantaged population of children at a time period when children in the control condition do not have alternative pre-k options. For these reasons, the estimated returns represent more of a proof of the principle that high-quality pre-k programs can produced positive economic benefits, rather than definitive evidence of the economic returns that would be expected from scaled-up programs.
"The Chicago CPC program is arguably a scaled-up part-day program operated by the Chicago Public School district and targeted to children in low-income neighborhoods ... However, the Chicago CPC program may also be viewed as exceptional because the program evaluation focuses on a cohort of children that attended the program in the early 1980s, with impacts that may not be replicated in today’s environment."Also, Karoly estimate the cost of a "school-day" pre-K program--that is, 6 hours per day--at about $8,000 per student: a little higher if the teachers are paid typical kindergarten wages.
3) One of the most careful and comprehensive studies of pre-K education published in 2013, evaluating the federal Head Start program, found a nearly complete fade-out of any positive effect of pre-K by third grade. The study said:
"In summary, there were initial positive impacts from having access to Head Start, but by the end of 3rd grade there were very few impacts found for either cohort in any of the four domains of cognitive, social-emotional, health and parenting practices. The few impacts that were found did not show a clear pattern of favorable or unfavorable impacts for children."4) I have started to wonder if effective interventions for disadvantaged children need to come considerably earlier than pre-K. Some evidence suggests that for a number of disadvantaged children, the gap in their cognitive skills emerges somewhere between 9 months and two years of age. Back in 2013, Richard V. Reeves, Isabel Sawhill and Kimberley Howard described this evidence in an essay called "The Parenting Gap," in which they pointed out that the federal government spends 25 times as much on Head Start as it does on programs targeted at parents of those same children for their first few years of life. In other work, Douuglas Almond and Janet Currie have argued that differences in cognitive abilities and other developmental measures can arise even before birth in "Killing Me Softly: The Fetal Origins Hypothesis," published in the Summer 2011 Journal of Economic Perspectives (where I work as Managing Editor).
In short, I worry that hard-headed analysis, together with a soft-hearted concern for disadvantaged children, should be pushing us toward interventions that will assist disadvantaged children well before they are old enough for a pre-K program.