Tc. Eley et al., Genetic and environmental origins of verbal and performance components of cognitive delay in 2-year-olds, DEVEL PSYCH, 35(4), 1999, pp. 1122-1131
The authors investigated the etiology of several measures of cognitive dela
y. Verbal (V) and performance (P) abilities were assessed in over 3,000 pai
rs of 2-year-old twins. Group-differences heritability for general delay (t
he lowest 5% of the V and P composite) was 35%. However, V and P delays con
sidered independently showed large differences in group heritability (77% f
or V vs. 40% for P). Specific delays with comorbid cases eliminated showed
an even greater difference in group heritability (78% vs. 22%, respectively
). The small sample comorbid for both V and P delay also yielded high group
heritability for bath V (77% ) and P (93%) scores. Shared environmental fa
ctors also differed in magnitude for V (20%) and P (41%) delays. Because th
e genetic and environmental origins of V and P delays in infancy differ, th
ey are better considered separately rather than combined into a composite m
easure of general cognitive delay.