The Evidence on Teaching

Family Environment and the Malleability of Cognitive Ability: A Swedish National Home-Reared and Adopted-Away Cosibling Control Study

Kendler, K. S., Turkheimer, E., Ohlsson, H., Sundquist, J., & Sundquist, K. · 2015

grade Btwin-adoptionindependentreplicatednumbers spot-checked
Sample
436 full-sibling pairs; 2,341 half-sibling pairs
Population
Swedish males with conscription-test IQ, where one sibling was adopted away and one reared by the biological parents.
Design
The cleanest available estimate of a whole-childhood rearing-environment effect on adult IQ, because it holds genetic background approximately constant within sibships.
Key findings
The key calibration study. Moving a child from a disadvantaged biological home to a screened adoptive home for their ENTIRE childhood raises adult IQ by ~4.4 points. That is the realistic ceiling on environmental manipulation of ability — and it means Abecedarian's ~+4.4 points sits AT that ceiling, not above it.
Counterfactual
The home-reared sibling raised by the biological parent(s) — a lower-SES, more disadvantaged rearing environment.
Genetic confound
Controlled by design (within-sibship, differing only in rearing).
Replication notes
Internally replicated in half-siblings. Provides the benchmark ceiling: whole-childhood upgrade of the rearing environment buys ~3-5 IQ points.
DOI / URL
10.1073/pnas.1417106112
Cite
BibTeXRIS

Effects

OutcomeMetricValueMeasureTimingVsHorizonClass
Adult IQ, adopted-away vs home-reared full siblingIQ points+4.41 (SE 0.75)standardizedadultnoneadulthoodg
Replication in half-siblingsIQ points+3.18 (SE 0.34)standardizedadultnoneadulthoodg
Per unit of rearing-parent educationIQ points+1.71 (SE 0.44)standardizedadultnoneadulthoodg

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