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 (...