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This study investigated whether genes affect language impairment to the same extent as they affect differences in language ability following up an earlier study of 579 four-year-old twins with low language performance and their cotwins ( ). The present study selected low-language twins from 6,963 pairs of twins from the Twins Early Development Study assessed for vocabulary and grammar by their parents at 2, 3, and 4 years of age. For impaired groups corresponding to the lowest scoring 5% and 10% at each age, twin concordances and model-fitting analyses indicated substantial genetic influence on the mean difference between affected children and the population ( h2 g), generally higher than for individual differences for the entire sample ( h2). [ABSTRACT FROM AUTHOR]
