Grover “Russ” Whitehurst of the Brookings Institution has spent decades studying early childhood education. Last month he offered a review of the evidence on the federal “Head Start” program targeted at low-income children and another on universal government Pre‑K programs.

Like most people who have chosen to work in this field, he is keen to find ways of improving educational outcomes for all children, and of helping disadvantaged children to catch up with their peers. Like only a very few, this goal has not lowered his standards of evidence. If there is a convincing rebuttal to Whitehurst’s essays, I haven’t seen it. And given the evidence as it exists today, I don’t expect to see such a rebuttal anytime soon.