The Environment in Raising Early Intelligence: A Meta-Analysis of the Fadeout Effect
Protzko, J. · 2015
grade Bmeta-analysisindependentunreplicatednumbers spot-checked
Sample
39 RCTs, 7,584 participants
Population
Children in RCTs that successfully raised IQ before kindergarten, then tracked IQ afterwards.
Design
RCT-ONLY growth-curve mixed-effects meta-analysis. Its key contribution is directly testing two rival mechanisms: reciprocal (controls catch up) versus unidirectional (the treatment group loses its gain).
Key findings
The most mechanistically precise fadeout study. IQ fadeout happens because the TREATED group loses its gain (b=-1.09), not because controls catch up (b=-0.21, ns). A striking illustrative case: LCPUFA supplementation raised IQ 0.44 SD at age ~4 with no environmental change at all, and had fully faded by age 7 — so fadeout is not merely 'returning to a disadvantaged environment'.
Counterfactual
Randomised control groups across the included trials.
Genetic confound
RCT base. Explicitly rules out genetic set-point, practice effects, lost knowledge, selective attrition and publication bias as sole explanations.
Replication notes
The mechanism it identifies is IQ-SPECIFIC. For achievement outcomes the mechanism is often the opposite — control-group catch-up (Bitler/Hoynes/Domina; the Building Blocks case in Bailey 2017). Record fadeout mechanism by outcome class, not globally.
DOI / URL
10.1016/j.intell.2015.10.006
Effects
| Outcome | Metric | Value | Measure | Timing | Vs | Horizon | Class |
|---|---|---|---|---|---|---|---|
| IQ trajectory, CONTROL group after treatment ends | b | -0.206 [-1.149, 0.737] — stable, ns | standardized | grade-3 | none | over-2yr | g |
| IQ trajectory, EXPERIMENTAL group after treatment ends | b | -1.09 [-2.149, -0.026] — significant DECLINE | standardized | grade-3 | none | over-2yr | g |
| Intervention duration as a moderator | moderator | no significant effect on magnitude or decay rate | standardized | grade-3 | none | over-2yr | g |
| Initial effect size vs decay rate | r | -0.298 (ns) — larger gains do not fade faster | standardized | grade-3 | none | over-2yr | g |
Cited by
- Fadeout — why early gains disappear, and what actually persistsstrong supportconf: highgc: low
- Perry Preschool and Abecedarian — what the famous studies actually establishmixedconf: mediumgc: low