Genetic and environmental influences on aspects of literacy and language in early childhood: Continuity and change from preschool to Grade 2
Byrne, B., Samuelsson, S., Olson, R. K., et al. · 2009
grade Btwin-adoptionindependentreplicated
Sample
International Longitudinal Twin Study (US/Australia/Scandinavia)
Population
Twin pairs across three instructional regimes differing in age/intensity of formal literacy instruction.
Design
Cross-national twin study exploiting differences in when/how intensively formal reading instruction begins.
Key findings
Formal instruction INCREASES the heritability of reading: pre-instruction h2 ranges 0.33-0.84 across countries but converges to ~0.8 after one year as the shared-environment component collapses. Genes do not cap instruction; instruction converts environmental variance into expressed genetic variance - the mechanism behind mean fadeout.
Genetic confound
The genetically-sensitive instruction datum itself.
Replication notes
The most direct evidence that instruction intensity raises expressed heritability; converges with Taylor 2010.
Effects
| Outcome | Metric | Value | Measure | Timing | Vs | Horizon | Class |
|---|---|---|---|---|---|---|---|
| Reading heritability, pre-instruction | h2 | 0.33 (Scandinavia, late instruction) to 0.84 (Australia, early) | standardized | cross-sectional | none | not-applicable | domain-skill |
| Reading heritability, after 1 year of formal instruction | h2 | converges to 0.79-0.83 everywhere; shared-environment c2 collapses | standardized | 1yr follow-up | none | not-applicable | domain-skill |
Cited by
- Systematic/explicit phonics vs whole language and balanced literacystrong supportconf: highgc: low