Perry Preschool, the most cited and most fragile trial
One hundred and twenty-three children, followed for forty years, supplied the central argument for investing in early childhood. The sample size is also the main objection.
The design
The programme ran in Ypsilanti, Michigan, from 1962. African-American children aged three and four, from low-income families, were allocated between an intensive preschool programme and no programme.
Allocation was close to random, which was rare for the period and explains the study's lasting place. Follow-up continued to age forty, then into the next generation.
The programme combined a few hours of daily classroom provision, with very few children per adult, and weekly home visits by teachers to the families, over one or two years depending on age at entry.
The reported results
Publications report durable gaps in completed schooling, employment, earnings and dealings with the justice system. The effect on cognitive scores, by contrast, fades quickly after school entry.
That contrast is one of the most discussed results in the field: what persists would not be what cognitive tests measure, an argument taken up by work on so-called non-cognitive skills.
The economic return calculation, which compares the programme's cost with the estimated gains for participants and society, did much for the study's reputation. It rests on valuation assumptions, notably about the cost of crime avoided, which weigh heavily on the result.
The objections
The sample is very small, around a hundred children split into two groups, which produces wide confidence intervals and makes results sensitive to a few individuals.
Initial allocation was altered along the way for practical reasons, which weakens randomisation. James Heckman and colleagues published reanalyses accounting for those departures, which maintain significant effects with more cautious estimates.
The transport problem
The programme was intensive, run by highly qualified staff, on a specific population, in a context where childcare alternatives were scarce. None of those elements is guaranteed in a large-scale rollout.
Evaluations of universal preschool programmes find smaller effects, which is expected: the comparison is no longer with no programme but with the other available forms of care. The Abecedarian trial, conducted in North Carolina from the 1970s, is often cited alongside it. It shares with Perry a small sample and an intensive programme, which makes their convergence instructive without settling the question of scale.
What the study licenses saying
It establishes that an intensive programme produced, in that context, durable gaps. It is a proof of possibility, solid within its frame.
It does not establish that a given programme, elsewhere and today, will produce the same effects. Distinguishing proof of possibility from prediction of effect is the point of method most often neglected when this work is taken up.
Key points
The trial covers about one hundred and twenty children, which makes estimates fragile despite the long follow-up.
The effect on cognitive scores fades, while gaps in schooling and employment persist.
It is a proof of possibility in a specific context, not a prediction of effect for a universal programme.
This article describes documented mechanisms and disagreements. It recommends no position, no party and no vote, and commits to no reading other than that of the work it cites.
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