Pre-kindergarten education is one of those policies that feels right to do-gooders. What are the actual consequences? According to an NBER report,
early education does increase reading and mathematics skills at school entry, but it also boosts children's classroom behavioral problems and reduces their self-control. Further, for most children the positive effects of pre-kindergarten on skills largely dissipate by the spring of first grade, although the negative behavioral effects continue.
The study does show advantages for disadvantaged children from attending pre-kindergarten.
I think it is always an uphill battle trying to bring evidence to bear in questions of social policy. Thanks to Tyler Cowen for the pointer.