Does anyone know where this claim actually comes from?
My best guess is that it comes from studies looking at the persistence of Head Start benefits. (For non-USAians, Head Start is essentially hothousing for low-SES kids - you put them in an academically enriched preschool so they enter K with the skills that high-SES kids have - knowing letters and numbers, knowing how to hold and open a book, etc.)
The short-term academic benefits of early education or preschool programs are well established. However, for many children the advantages bestowed by early education fade by the second or third year of formal schooling, as their counterparts who did not attend early education programs �catch up� (Barnett, 1995; Lazar, Darlington, Murray, Royce, & Snipper, 1982; McKey et al., 1985).
