Abstract
Purpose Early language and speech acquisition can be delayed in twin children, a twinning effect that diminishes between 4 and 6 years of age in a population-based sample. The purposes of this study were to examine how twinning effects influence the identification of children with language impairments at 4 and 6 years of age, comparing children with specific language impairment (SLI) and nonspecific language impairment (NLI); the likelihood that affectedness will be shared within monozygotic versus dizygotic twin pairs; and estimated levels of heritability for SLI and NLI. Twinning effects are predicted to result in elevated rates of language impairments in twins. Method The population-based twin sample included 1,354 children from 677 twin pairs, 214 monozygotic and 463 dizygotic, enrolled in a longitudinal study. Nine phenotypes from the same comprehensive direct behavioral assessment protocol were investigated at 4 and 6 years of age. Twinning effects were estimated for each phenotype at each age using structural equation models estimated via diagonally weighted least squares. Heritabilities were calculated for SLI and NLI. Results As predicted, the twinning effect increased the percentage of affected children in both groups across multiple language phenotypes, an effect that diminished with age yet was still not aligned to singleton age peers. Substantial heritability estimates replicated across language phenotypes and increased with age, even with the most lenient definition of affectedness, at -1 SD. Patterns of outcomes differed between SLI and NLI groups. Conclusions Nonverbal IQ is not on the same causal pathway as language impairments. Twinning effects on language acquisition affect classification of 4- and 6-year-old children as SLI and NLI, and heritability is most consistent in the SLI group. Clinical practice requires monitoring language acquisition of twins to avoid misdiagnosis when young or a missed diagnosis of language impairments at school entry.
Highlights
W orldwide, the proportion of babies born as twins is increasing
This study focuses on twin children with language impairments, in the form of specific language impairment (SLI) or nonspecific language impairment (NLI), following classic experimental methods of defining affectedness (Tomblin et al, 1997)
This study aims to fill these gaps in the research base, using consistent criteria across the SLI and NLI groups, in a longitudinal sample of twins at 4 and 6 years of age
Summary
W orldwide, the proportion of babies born as twins is increasing. In the United States in 2009, one in every 30 babies (3.3%) born was a twin, compared to one in every 53 babies (1.8%) in 1980 (Martin et al, 2012). The increase is due to two factors: the tendency for. The potential scientific informativeness of twins has long been recognized. The most widely used methodology for evaluating the role of genetics and environment in language and cognition, including their development and impairments, has been the twin study, which uses—rather than studies—twins. There is a much smaller literature focused on twins, which has typically
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