On February 3, 2014, I spoke at a conference on early childhood education sponsored by the Education Writers Association. Later, the conference heard from many other speakers, including Russ Whitehurst of the Brookings Institution.

Whitehurst expressed uncertainty about whether early childhood education has lasting impacts. Journalists listening to Whitehurst might conclude that there is no research consensus on the impacts of early childhood education.

In my opinion, there is a research consensus that high-quality early childhood education can have lasting impacts. Whitehurst’s perspective represents a minority of researchers who dissent from what the bulk of the research shows.

Whitehurst’s position is based on emphasizing two studies, while downplaying all other research studies. He emphasized the Head Start randomized control trial and the Tennessee pre-K randomized control trial. These studies found immediate impacts of pre-K, which faded over time.

However, most good studies do find lasting impacts of early childhood education. These studies include the two best-publicized randomized control trials of early childhood education, the Perry Preschool study and the Abecedarian study.

Whitehurst’s presentation implied that Perry and Abecedarian are less relevant because they were done a long time ago. But early childhood studies that look at adult impacts up to 36 years later necessarily must have been started a long-time ago.

Whitehurst also implied that the two studies are less relevant because they provided services that differ greatly from what early childhood education programs are today. But the Abecedarian program is quite similar in services offered to the present-day Educare program.

And Perry Preschool does not differ in kind from many pre-K programs today. It had lower class sizes than most of today’s programs, and provided services for ages 3 and 4 rather than just age 4.  But it is similar to many of today’s programs, for example programs in Tulsa, Boston, and the Chicago Child-Parent Center in using certified teachers paid public school wages. And Perry was only a half-day program whereas Boston’s program is full-day and Tulsa’s program includes many full-day centers.

Perry’s program had an estimated 19% effect in increasing adult earnings.  The larger class size and one-year nature of many of today’s pre-K programs might somewhat decrease adult earnings impacts, while a full-day program might increase adult earnings impacts. Based on what we know about how class size, full-day vs. half-day, and two years versus one year affects impacts, we might think that many of today’s pre-K programs might have somewhat lower adult earnings impacts than 19%.  But even an impact of 6% or more (which is what we tend to find in the studies of pre-K reviewed below) would have a very high ratio of benefits to costs.

More importantly, there are many other studies than Perry and Abecedarian that show that early childhood education can have lasting impacts.  These include the Infant Health and Development Program, also a randomized control trial, as shown in a recently published study by Duncan and Sojourner.

These other studies also include many other studies which are not randomized control trials, but which do have very good comparison groups.  These studies are “natural experiments”, in which whether or not a child participates in early childhood education is determined by the accidents of geography or age or other circumstances that are likely unrelated to unobserved characteristics of the child or family. These natural experiments provide good evidence because the lasting differences between the treatment group and the comparison group are most plausibly attributed to the program’s true causal effects, as there is no good reason to think that there are significant unobserved pre-existing differences between the treatment group and the comparison group. Natural experiments that show lasting impacts of early childhood education  include the many Chicago Child-Parent Center studies, studies of Head Start by Deming , Currie et al. and Ludwig et al. , and a study of North Carolina’s early childhood programs by Ladd et al.

Many of the research studies that find lasting impacts of early childhood education also find that cognitive test score impacts fade over time.  Test score fading is found in Perry Preschool, the Abecedarian program, the Chicago Child-Parent Center program, and in Deming’s study of Head Start. Despite test score impacts that fade during the K-12 years, all these studies find large impacts of early childhood education on adult outcomes, with these impacts being “large” in the sense that they either directly show large percentage adult earnings impacts, or have educational attainment impacts that would predict large percentage adult earnings impacts.

Therefore, contrary to the impression left by Whitehurst’s presentation, there is a significant research basis for believing that even if there is fading in the Head Start randomized control trial and the Tennessee pre-K randomized control trial, there may well be later large effects on adult outcomes.  The most plausible theory for test score fading but long-term adult benefits is that early childhood education leads to lasting impacts on “soft skills” (social skills, character skills). These lasting soft-skill effects are extremely important in determining adult outcomes in higher educational attainment and higher employment rates and wage rates.

In the case of Head Start, the 3^rd grade cognitive test score impacts, while mostly statistically insignificantly different from zero, are large enough that they would predict over a 1% impact on adult earnings, which would be a lot of money over a person’s entire working career. If these are faded impacts that significantly underpredict adult earnings impacts, the true adult earnings impact could be much greater. In addition, the confidence interval on these 3^rd grade test score impacts is large enough that while it cannot rule out zero impacts, it also cannot rule out test score impacts that would predict a 2 to 3% increase in adult earnings. Therefore, the Head Start impacts are consistent with both zero test score impacts at third grade, and with test score impacts that would be large enough to be relevant for policy purposes.

The research literature also suggests that the early post-program impacts of early childhood education on test scores are better predictors of long-term impacts on adult earnings than are later, faded test score impacts. This finding occurs in the Chicago CPC study, the Abecedarian study, the Perry study, and Deming’s Head Start study. This finding also occurs in Chetty et al.’s study of the adult earnings impacts of higher kindergarten quality.

Therefore, the many recent “regression discontinuity” studies of state and local pre-K that show large effects on kindergarten entrance test scores adds some additional support to the notion that pre-K has large impacts. (These studies include studies by the National Institute for Early Education Research in seven states, and studies of Tulsa, Boston, and Kalamazoo.) These studies do not directly show lasting impacts of pre-K. But they show much larger immediate impacts of many state and local pre-K programs than are found in the Head Start randomized control trial or the Tennessee randomized control trial.  Based on the studies that show that early test score impacts predict long-term adult earnings effects, there is good reason to think that these state and local pre-K programs will significantly increase adult earnings.

We should also recognize that even if randomized control trials are the “gold standard” for research evidence, natural experiments meet a good “silver standard” that should also be considered in deciding on the research consensus. No one study is perfect in methodology, and therefore we should consider what the bulk of studies show. Furthermore, most studies look just at one program, and therefore if we want to know whether early childhood education in general tends to work, we also need to look at the bulk of studies, rather than just one study of one program.

Like most studies, the Head Start randomized control trial and the Tennessee randomized control trial have some methodological limitations. The Head Start experiment had an unusually large number of members of the control group that participated in some other pre-K program (about half), therefore it is better interpreted as an experimental study of whether Head Start as of 2003-04 was on average better in its test score impacts than the average quality of other pre-K alternatives, including state pre-K programs.

The Tennessee study had large and differential attrition from the treatment group and the control group. For example, in the first cohort, the analyses were based on data from only 46% of the treatment group and 32% of the control group. Although the original treatment and control groups might be similar in unobserved characteristics, it is quite possible that the much smaller group on which the research was largely based may have large differences among the treatment and control group, which raises questions about whether or not the study meets the gold standard. The researchers tried to control for observable differences among the two groups, but obviously it is impossible to statistically control for unobservable differences, which is what a randomized control trial is trying to do, and what a natural experiment is argued to do.  There are some signs that the full sample showed larger impacts on kindergarten retention than the smaller sample after attrition, which suggests that fears of possible biases may be warranted.

In addition, neither Head Start as of 2003-04 nor the Tennessee pre-K program necessarily represent the impact of the highest quality pre-K programs. As mentioned, many regression discontinuity studies of state and local pre-K programs show higher immediate impacts than Head Start (see also Wong et al.). Tennessee’s pre-K program is judged by the National Institute for Early Education Research to spend $2,000 per child less than is thought to be a reasonable amount per child to facilitate quality pre-K services.

Overall, the bulk of the research evidence from many studies has convinced most researchers that high-quality early childhood education can have large and lasting impacts.  The Head Start randomized control trial and the Tennessee randomized control trial do not provide additional evidence that supports the effectiveness of early childhood education. But contrary to Whitehurst and a minority of researchers, these two studies are insufficient to overturn the bulk of the research evidence from multiple studies of a wide variety of programs.