Do genetic factors explain the link between language and school readiness in toddlers?

The positive association between language skills and school readiness has been well established (Margo et al., 2011). Early vocabulary and later school readiness both show small, but significant, heritability with shared and... [ view full abstract ]

The positive association between language skills and school readiness has been well established (Margo et al., 2011). Early vocabulary and later school readiness both show small, but significant, heritability with shared and nonshared environmental influences contributing the most to their variances (Foget-Dubois et al., 2009). Moreover, the covariance between vocabulary and school readiness are due only to environmental (shared and nonshared) influences. However, this research relied on parent-report to assess vocabulary, which may have inflated shared environmental influences due to rater biases.  To date, no study has examined the early relation between language and school readiness assessed through behavioral assessments.  We investigated the genetic and environmental contributions to individual differences in vocabulary and school readiness and their overlap in 3-year-old twins (123 MZ, 189 DZ).  School readiness was measured using the Bracken School Readiness Assessment (BSRA; Bracken, 2000) and receptive vocabulary was measured using the NIH Toolbox Picture Vocabulary Test (PVT; Gershon et al., 2013).  We found a moderate phenotypic correlation between the BSRA and PVT (r=0.38, p<0.05).  Cholesky model fitting analyses showed genetic influences accounting for 29% (CI 9%-51%) of the variance for the BSRA and 31% (CI 6%-55%) for the PVT.  Shared environment influences were significant for the BSRA (41%; CI 20%-57%), but not for the PVT (14%; CI 0%-35%).  The remaining variance for BSRA and PVT was explained by nonshared environmental influences.  Overlap between BSRA and PVT was explained entirely by genetic effects, and these genetic effects overlapped completely (r_G=1.00; CI 0.49-1.00).  Unlike previous findings, we found that the relation between school readiness and language abilities was due to shared genetic influences and not environmental influences.  This could be due to the different measures used in assessing language as our current study relied on behavioral assessments and not parent-report.