67. Reznick, J. S., Robinson, J., & Corley, R. (1997). A longitudinal twin study of intelligence in the second year. Monographs of the Society for Research in Child Development.

The measures used are the Bayley Mental Development Index (MDI), its subdivision into three process-oriented constructs to measure nonverbal ability, expressive language, and receptive language, and laboratory tests of word comprehension, visual attentiveness, and memory for locations. Scores on most measures increase with age and are reasonably stable for individuals, with greater stability late in the second year. Analyses reveal various patterns of change, earlier change for language than for the nonverbal construct, and earlier change for girls on verbal measures and boys on nonverbal measures. Analyses indicate genetic effects on the MDI at each age, on longitudinal continuity and change, on nonverbal abilities at each age, and on expressive and receptive language late in the second year. Shared environmental influences are seen for the MDI at 20 and 24 months, on longitudinal continuity, and for most of the phenotypic similarity and continuity and change in expressive and receptive language. Comparisons among measures confirm the predictive significance of receptive language through the second year and the importance of genetic influences on changes at 20 months.