This study of 4-year-old twins investigated the genetic and environmental origins of comorbidity between language impairment and nonverbal ability by testing the extent to which language impairment in one twin predicted nonverbal ability in the co-twin. Impairment of language ability was defined as scores below the 15th percentile on a general language scale derived from a battery of diverse language tests. Four hundred thirty-six children, members of 160 monozygotic (MZ) and 131 same-sex dizygotic (DZ) twin pairs, were identified as language impaired. Language-impaired probands also suffered significant impairments in nonverbal ability. DeFries-Fulker extremes analysis showed evidence for substantial genetic mediation of the phenotypic relationship between language impairment and poor nonverbal ability in that language problems in one twin predicted poor nonverbal ability in the co-twin, much more so for MZ twins than for DZ twins. This finding held even when we excluded those children with language impairment whose nonverbal score indicated general cognitive delay. These results point to a general genetic factor that includes both language and nonverbal problems.
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