A DEVELOPMENTAL-GENETIC ANALYSIS OF CONTINUITY AND CHANGE IN THE BAYLEY-MENTAL-DEVELOPMENT-INDEX FROM 14 TO 24 MONTHS - THE MACARTHUR LONGITUDINAL TWIN STUDY

A developmental-genetic model was fitted to Bayley Mental Development Index (MDI) data to address questions concerning the origins of indivi dual differences in MDI performance and the origins of change and cont inuity during infancy. More than 350 pairs of identical and same-sex f raternal twins were studied longitudinally at 14, 20, and 24 months of age. There was substantial genetic continuity of general cognitive ab ility from 14 to 24 months, but significant new genetic variation also appeared at 24 months. Shared family environmental influences were gl obal across all three ages and not time-specific. Finally, nonshared e nvironmental influences did not contribute to the observed continuity of general cognitive ability, only to change.