The Hamilton Project has released a useful e-book that presents evidence on selected anti-poverty policies. This includes some discussion of pre-K programs, by Elizabeth Cascio and Diane Whitmore Schanzenbach.

The Cascio/Schanzenbach chapter argues for expansion of high-quality targeted pre-K.  My own view, as I have stated previously, is in favor of high-quality universal pre-K. What is the evidence for and against each position?

Part of the issue is political. I simply do not think there will ever be sufficient political support for targeted pre-K programs to enable large-scale access of the poor and near-poor to high-quality pre-K. Therefore, from a political perspective, Cascio/Schanzenbach are supporting a policy proposal that will never be fully implemented.

But I also think there are good economic reasons to support universal pre-K over targeted pre-K. Part of the issue is what evidence one finds more convincing.

Cascio/Schanzenbach rely on their own evidence, from a prior paper analyzing test score effects in Georgia and Oklahoma, to argue that universal pre-K mainly benefits the disadvantaged. The problem with this evidence is that it relies on comparisons between what happens in two states and what happens in other states. There are many unobservables that affect test scores in any two states. These unobservables can bias estimates. Furthermore, these unobservables do blow up standard errors when one fully allows for them. As they acknowledge in their own prior paper, estimation procedures that fully allow for the many unobserved “shocks” that affect state test scores find that most of their estimated effects have very big standard errors, so it is hard to reach definitive conclusions.

It is a tempting strategy to use states as “laboratories of democracy”. The estimation problem is that if one only has two “test subjects” (states) in the “treatment group”, estimation errors tend to be very wide.

In contrast, I would point to what I would argue is stronger evidence, from “regression discontinuity” studies of Boston and Tulsa. (Cascio/Schanzenbach have a footnote referring to the Boston study, but they don’t mention the Tulsa study.) This evidence indicates that these universal pre-K programs have effects on kindergarten entry test scores for middle-class children that are 70% (Boston) to 90% (Tulsa) of effects for lower-income children.

In my opinion, regression discontinuity studies provide “silver standard” evidence of effects of pre-K. These studies are NOT random assignment experiments, which would provide stronger, “gold standard” evidence. However, these regression discontinuity studies seem unlikely to be biased by unobservable differences between the “treatment group” (the children who have completed pre-K, and who are at kindergarten entrance) and the “control group” (the children just entering the pre-K program). Both groups of students either are just entering or have recently finished the same pre-K program, so the two groups do not differ in factors causing families to select or not select a pre-K program, or in factors that might affect how program procedures might lead to the programs selecting certain types of families.

The regression discontinuity procedure is essentially to look at the differences between the two groups of students in scores on the same tests, and then to look at how scores vary with age to control for the fact that the treatment group is on average one year older. We expect to see that the scores will go up with age, and if pre-K has an effect, will show a “jump” (a discontinuity) at the age cut-off that separates children who are just a little too young to yet attend kindergarten, and who are entering pre-K, and children a few days older who are just old enough to enter kindergarten, and who participated in pre-K the previous year.  (See my paper with Gormley and Adelstein for more extended discussion of the regression discontinuity model applied to pre-K.)

In addition to this “silver standard” evidence, we have one random assignment study, of Utah, that provides “gold standard evidence” that pre-K has positive effects for middle-class children.

I would regard the Boston/Tulsa, and Utah studies, as providing stronger evidence than comparisons between Georgia/Oklahoma and other states, largely because of the many unobservable variables that can cause test scores in a state to fluctuate. Having said that, is the evidence that pre-K benefits middle-class children as strong as the evidence that pre-K benefits lower-income children? No, it is not, largely because pre-K for lower-income children has been subject to many high-quality studies, both random assignment and others with good comparison groups, which show that pre-K benefits lower-income children. In contrast, there are fewer studies on pre-K services to middle-class children.

Here is where we get into the highly subjective issue of where the burden of proof should be. One position is to say, if there is no strong and overwhelming evidence that pre-K benefits middle-class children, but that there is such evidence for lower-income children, we should only favor targeted services. We shouldn’t favor a universal program unless there is very strong evidence for benefits to middle-class children.

The other position is to say, given that there is very strong evidence of pre-K benefits for lower-income children, and some good evidence for its benefits for middle-class children, it is more likely than not that pre-K programs benefit middle-class children. And one could argue that this is more than enough evidence to move ahead.

Let’s think through the benefits and costs of moving ahead with universal programs. There is some evidence that this will benefit middle-class children, although not “proof”. There is some evidence that this will increase political support for the programs. At the very least such programs provide some help to working-class voters facing various economic issues. And pre-K programs including middle-class children may have more favorable peer effects that will increase program quality for lower-income children. In my view, the benefits of moving ahead with universal programs outweigh the risks.

In contrast, if we just move ahead with targeted pre-K for lower-income children, we may be missing an opportunity to have more significant effects on overall labor force quality by also assisting middle-class children. We lose any opportunity for positive peer effects from having income-mixed classrooms. And we are setting up a program that is doomed to difficulties in eliciting political support, particularly at the state and local level. There are risks involved in supporting targeted programs over universal programs.

Targeted vs. universal pre-K programs is a debate in which we must go beyond a narrow economics debate over what effects are statistically significant, to a debate over the preponderance of the evidence, and debate over what policy approach is best suited for making progress in a real-world political environment.