Longitudinal analysis of the genetic and environmental influences on components of cognitive delay in preschoolers
Eley TC., Bishop D., Dale P., Price TS., Plomin R.
The etiology of verbal and performance delays were investigated in a large population sample of twins at 2 and 3 years (N = 2,449 pairs). These data replicate and extend earlier analyses at 2 years (T. C. Eley et al., 1999). Several ways of defining delay were compared, selecting from the lowest 5% of the sample on both verbal (V) and performance (P) measures. V delay with or without P delay was highly heritable (.93 and .62, respectively), whereas P delay without V delay was less heritable (.29), with substantial shared environment influence (.48). Longitudinal genetic analyses indicated substantial heritability of V delay from 2 to 3 years, especially in combination with P delay (.48 and .81, respectively). Similarly, genetic influence on continuity of P delay without V delay was much lower (.22). These results suggest that it is useful to consider the strong genetic contribution to verbal delay regardless of nonverbal impairment in preschoolers.