President Obama says that the science is clear. Greg J. Duncan and Katherine Magnuson are not so sure.

We find that the evidence supports few unqualified conclusions. Many early childhood education programs appear to boost cognitive ability and early school achievement in the short run. However, most of them show smaller impacts than those generated by the best-known programs, and their cognitive impacts largely disappear within a few years. Despite this fade-out, long-run follow-ups from a handful of well-known programs show lasting positive effects on such outcomes as greater educational attainment, higher earnings, and lower rates of crime. Since findings regarding

short and longer-run impacts on “noncognitive” outcomes are mixed, it is uncertain what skills, behaviors, or developmental processes are particularly important in producing these longer-run impacts