Adoption and Cognitive Development: A Meta-Analytic Comparison of Adopted and Nonadopted Children's IQ and School Performance
van IJzendoorn, M. H., Juffer, F., & Klein Poelhuis, C. W. · 2005
grade Cmeta-analysisindependentreplicated
Sample
62 studies, N=17,767
Population
Adopted children compared with non-adopted siblings and with peers left behind.
Design
Meta-analysis of adoption studies — the largest synthesis of the natural experiment that most cleanly separates rearing from heredity.
Key findings
The strongest single body of adoption evidence, and it cuts both ways: moving children out of deprivation raises IQ substantially, yet adoptees end up indistinguishable from the biological children of their adoptive families on IQ. Environment matters most at the deprived margin and much less across the normal range.
Genetic confound
Adoption design; the within-adoptive-family comparison is the key contrast.
Replication notes
The largest adoption meta-analysis available; converges with Kendler (2015) on the size of the achievable environmental gain.
DOI / URL
10.1037/0033-2909.131.2.301
Effects
| Outcome | Metric | Value | Measure | Timing | Vs | Horizon | Class |
|---|---|---|---|---|---|---|---|
| Adoptees vs their environment-of-origin peers (left behind) | direction | adoptees score substantially higher — adoption raises IQ relative to the deprived counterfactual | standardized | adult | none | adulthood | g |
| Adoptees vs non-adopted siblings IN THE ADOPTIVE FAMILY | difference | adoptees do NOT differ on IQ | standardized | adult | none | adulthood | g |
Cited by
- The early home environment — what parents can and cannot causally movemixedconf: highgc: low