A recent opinion piece by David Kirp in the New York Times argued that it makes no sense to put low-income children in income-segregated pre-K programs, as we do in the Head Start program, because of the importance of classroom peer effects. If low-income children learn more if more of their peers are from a variety of backgrounds, then pre-K programs will be more effective in closing achievement gaps if they are income-integrated programs. This does not necessarily require universal free pre-K (one could imagine some sort of voucher program for low-income children, or some sort of sliding scale fees for a universal pre-K program), but it is one argument in favor of real-world universal programs such as the program in Oklahoma, and against common targeted programs in which low-income children are restricted to their own pre-K classrooms.

(David Kirp is a former newspaper editor and current public policy professor at Berkeley who has written two very good books that directly address early childhood education, The Sandbox Investment, and Kids First. His most recent book, Improbable Scholars, looks at an inner-city school district that he argues has achieved success through a variety of policies and practices that include early childhood education. )

Kirp cites one study of Connecticut preschools to support his opinion piece, by Schechter and Bye (2007). To my knowledge, there are four other preschool studies that look at peer effects, by Henry and Rickman (2007), Mashburn et al. (2009),  Justice et al. (2011), and Reid and Ready (2013). What do we find from these studies?

(1)    Only Schechter/Bye and Reid/Ready directly look at peer income effects. The other studies look at effects of peer pre-existing skills on student learning during pre-K. All of these studies show effects of peers in raising learning during pre-K.

(2)    Henry and Rickman’s results suggest that the magnitude of this effect averages about a 20% spillover: if my peers at the beginning of pre-K have 50% higher skills, we would expect my test scores to be 10% higher at the end of pre-K (10% = 20% of 50%), holding all other pre-K characteristics constant.

(3)    The studies have mixed evidence on whether greater integration of pre-K of children with different characteristics will increase overall pre-K effects for the entire population.

Let me elaborate on that last point. Peer effects at least potentially go in both directions: students with stronger initial skills may have peer effects on their initially less-skilled peers, and students with weaker initial skills may have peer effects on their initially more-skilled peers. If these peer effects are completely the same for all types of children, in all types of classrooms, then peer effects would not lead greater income integration or skills integration to increase overall pre-K performance.

For example, suppose we consider two alternatives: completely segregated classrooms by income and skill level, and completely mixed classrooms by income and skill level. If we go from the segregated to the mixed classroom situation, the lower income or lower-skill students benefit from the influence of their higher income or higher-skill peers. But the upper income or higher-skill students might lose the same amount from the influence of their lower income or lower-skill peers.

The case for income and skills integration is that these peer effects are ASYMMETRICAL, that is differ either across different types of students, or at different levels of integration. For example, suppose that lower-income or lower-skill students on average are very influenced by their peers, but upper-income or higher-skill students on average are not so influenced by their peers. This might well be plausible. One could imagine that learning depends upon the richness of language one hears at school, at home, and at play. If middle-class or middle-skill children already have a higher likelihood of having been exposed to such rich language outside of school, perhaps they are less dependent on hearing such language at school. But lower-skill or low-income children might be more dependent, on average, on hearing such language at school.

(A word might be appropriate about the dangers of generalizing about a group. I fully recognize that there is great diversity of children within any group we might define based on income or some test. The peer effect patterns I am referring to might be tendencies for the average child in the group, and may not be at all true of any individual child.)

As another example, peer effects might differ across classrooms.  For example, perhaps an income or skills-integrated classroom is far better in its “peer effects” than a classroom with 100% low-income or initially lower-skilled children, but perhaps the well-integrated classroom is similar in its peer effects to a classroom with 100% higher-income or initially higher-skilled children. This also seems plausible as a hypothesis. In other words, there might be some threshold or tipping effects of different levels of income and skills integration.

When I say the effects from the current research is mixed, what I mean is that only two studies specifically look at this symmetry, and they find different results. Mashburn et al. (2009) find some evidence that peer effects are larger for children with higher initial skills. On the other hand, Justice et al. (2011) find some evidence that peer effects are larger in going from 100% low-skill classrooms to integrated classrooms than they are in going from integrated classrooms to 100% high-skill classrooms.

However, there is other evidence that also bears on this issue. Both Tulsa and Boston run pre-K programs that include some middle-class children as well as low-income children. For both these cities’ pre-K programs, although there are plenty of middle-class children, they are in a minority: 25% of the Tulsa pre-K children are ineligible for a free or reduced price lunch, and 31% of the Boston pre-K children are ineligible for a free or reduced price lunch.

The evidence from both Tulsa and Boston suggests that whatever negative peer effects MIGHT occur for middle-class children from being in a pre-K program that includes a substantial majority of children eligible for a free or reduced price lunch, these peer effects do not prevent middle-class children from gaining substantially from income-integrated pre-K programs. The gains from pre-K for middle class children in test score percentiles in Tulsa are about 90%of the test score gains for lower-income children; the gains from pre-K for middle class children in Boston are about 70% of the test score gains for lower-income children. In both cases, the predicted dollar effect of these pre-K programs on future adult earnings for middle-class children are at least double program costs.

In other words, whatever academic uncertainty there is about the nature of peer effects, universal income-integrated pre-K programs, if run in a high-quality fashion, appear to be able to achieve substantial benefits for both the poor and the middle class.