One of the honest promises put forth by Amendment 66 supporters is that a portion of the funds will go to expanding preschool access for low-income families. The publicly-funded Colorado Preschool Program touts research that shows it’s making a positive difference.

But a new Time column by Kay Hymowitz (H/T Joanne Jacobs) reminds us what the research says about the true limitations of Early Childhood Education:

It’s true that good preschools raise the math and reading scores of disadvantaged kids. The problem is that the gains are almost always temporary. Study after study of every kind of program since Head Start first came on line in the 1960’s to recent state wide programs in Georgia and Oklahoma has concluded that, with the lonely exception of third grade boys’ math scores in Tulsa, cognitive gains “fade out” by third grade, probably because subpar schools and an unsupportive environment at home were unable to help pre-K kids take advantage of those gain.

Hymowitz goes on in the column to deconstruct the most famous preschool research cited by advocates, the Perry Project, and the marginal advantages that it offered. Those in the program “grew up to become low skilled, low income single parents, less costly to society than others without their early educational advantage, but equally likely to raise children who would cycle back into poverty.”

Regarding the exceptional good news from Tulsa, the Brookings Institution’s Russ Whitehurst and George Mason University professor David Armor found the research methods lacking. The evidence isn’t there, they say, to press forward with a federal universal pre-K program — especially since the Tulsa highlights are little better than the 48-year disappointment known as Head Start.

To be clear, preschool funding by no means should be a make-or-break issue for the billionaire-backed Amendment 66. But when so much doubt lingers about one of the alleged strong points of the tax hike initiative, it should give enough pause to re-examine the painful price tag and unfair redistribution at the proposal’s heart.