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 co-twins (Viding et al., in press). The present
study selected low-language twins from 6,963 pairs of twins from the Twins Early Development Study (TEDS) 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 (h^2 g), generally higher than for individual differences for the entire sample (h^2 ).
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