Grover J. “Russ” Whitehurst writes,

I am concerned that preschool education has become like organic food — a creed in which adherents place faith based on selective consideration of evidence and without weighing costs against benefits. The result may be the overselling of generic preschool education as a societal good and a concomitant lack of attention to the differential impact of different types of preschool experience on different categories of children. So just as some but not all foods grown under some but not all organic conditions may be worth their price because of their extra nutritional benefits and lower environmental impacts, some but not all children exposed to some but not all preschool programs may experience lasting benefits. And because preschool education like organic food is expensive, it pays to know what works best, for whom, under what circumstances.

He proceeds to cite Head Start as an example of a program with high costs and negligible long-term benefits. See also Timothy Taylor.

In a follow-up post that discusses research into other programs, Whitehurst writes,

This thin empirical gruel will not satisfy policymakers who want to practice evidence-based education. Their only recourse if they have to act is to do so cautiously and with the awareness that they are going to make some mistakes and need to be in a position to learn from them. They and the general public need to be wary of the prevailing wisdom that almost any investment in enhancing access to preschool is worthwhile. Some programs work for some children under some conditions. But, ah me, which programs, children, and conditions?

Nicholas Kristof also talks about research into cost-effectiveness of anti-poverty programs. He wants to see taxpayers spend more money on the pre-school programs that have been shown to have success. But he does not want to spend any less on worthless programs. He even insists on hanging on to Head Start.