In my new book, From Preschool to Prosperity, one issue I discuss is how the returns to high-quality early childhood programs vary with what child ages are targeted by the program. One popular notion is that the earlier the intervention, the better. Although this notion has a kernel of truth, the research evidence on the whole supports the notion that programs at pre-school ages tend to have the highest benefit-cost ratio.

The kernel of truth is that earlier interventions seem to have the potential for more dramatic effects on adult outcomes. For example, evidence from the Abecedarian experiment, which provided full-time child care and pre-K from birth to age 5, suggests that such services can boost adult earnings by over 25%. (The Abecedarian program is similar to today’s Educare program.)

But from a benefit-cost standpoint, earlier interventions also cost more per child. The younger the child, the more interventions have to have smaller class sizes or be one-on-one in order to be high quality. This lowers the benefit-cost ratio.

For example, the evidence suggests that one-year of full-day pre-K at age 4 can raise future earnings by 10%. This increases the present value of future earnings for former child participants by about $50,000, at a cost of around $10,000, for a benefit-cost ratio of over 5 to 1.  Extending this program to younger ages by adding in all the services of an Abecedarian-style program from birth to age 4 would increase the future earnings benefits to 26%, or two-and-a-half times as much.  The increase in the present value of future earnings is around $130,000, which is a much more dramatic increase than for one-year of preschool. But the cost of this program is almost 9 times as much, or almost $90,000 for the five years of full-time child care and pre-K.  The benefit-cost ratio in terms of benefits for children is around 1.5, which is considerably lower than for one year of pre-K.  An Abecedarian program has much larger child care benefits for parents, which would more than double the earnings effects of the program. But even with these parental earnings benefits, preschool at age 4 still has a somewhat higher benefit-cost ratio than full-time child care and pre-K from birth to age 5.

The reason that one-year of pre-K has a higher bang for the buck than the earlier child care interventions is in part that child care has to have smaller group sizes than preschool to be high-quality. High-quality pre-K can have class sizes of 17 children to 2 teachers, or even 20 children to 2 teachers, class sizes that would be infeasible for younger age programs that aspire to high quality. Preschool at ages 3 and 4 appears to be an age range in which children are still malleable enough to significantly affect their future life-course, but children are old enough that they can be dealt with in large enough groups to significantly lower program costs per child.

On the other hand, if one truly wants very large effects on a child’s future life course, these earlier interventions can do more than just one year of pre-K. “Bang for the buck” is not the only criteria for evaluating programs. We also want to have large impacts.

Both pre-K and earlier interventions make sense. Pre-K is an extremely cost-effective approach to affecting a child’s future. But if we want to have truly large effects on bringing children out of poverty, we need to supplement pre-K with earlier high-quality interventions.