Last night, President Obama made some pretty grand claims for the power of preschool:

Every dollar we invest in high-quality early education can save more than seven dollars later on—by boosting graduation rates, reducing teen pregnancy, even reducing violent crime. In states that make it a priority to educate our youngest children, like Georgia or Oklahoma, studies show students grow up more likely to read and do math at grade level, graduate high school, hold a job, and form more stable families of their own

The president is proposing a national preschool entitlement, focused on low- and middle-income families. (Though his actual preschool proposal is only slightly more detailed than what he mentioned in his speech.)

If only we had some kind of large scale well-tracked pilot program that could give us some information about whether that is a good idea. Oh wait! We do! It's called Head Start, the $8 billion federal program catering to more than 1 million low-income kids.

Better still, the federal government has done a huge study, tracking 5,000 kids and comparing them to kids who did not have access to Head Start.

The findings are not impressive. A 2010 analysis of that group found that the cognitive, health, parenting, and social benefits of the program had vanished by first grade. And a 2012 look at the third grade outcomes was even less heartening, with no discernible academic gains and teachers reporting slightly more behavioral problems in the Head Start kids.

Even if Georgia and Oklahoma have managed to formulate slightly more effective programs (Georgia is experimenting with a voucher-like system), there's still the larger evidence of the performance of American public schools overall in the last couple of decade. Spending is way, way up while academic results remain flat.

The current performance of Head Start and public schools overall is not exactly making a compelling case that we should spend hundreds of billions more dollars to shovel kids into this system earlier.