Heritability of specific language impairment depends on diagnostic criteria

Heritability estimates for specific language impairment (SLI) have been inconsistent. Four twin studies reported heritability of 0.5 or more, but a recent report from the Twins Early Development Study found negligible genetic influence in 4-year-olds. We considered whether the method of ascertainment influenced results and found substantially higher heritability if SLI was defined in terms of referral to speech and language pathology services than if defined by language test scores. Further analysis showed that presence of speech difficulties played a major role in determining whether a child had contact with services. Childhood language disorders that are identified by population screening are likely to have a different phenotype and different etiology from clinically referred cases. Genetic studies are more likely to find high heritability if they focus on cases who have speech difficulties and who have been referred for intervention

Assessing children who cannot be 'tested'

The Scottish Low Birth weight Study was set up to follow the developmental progress of all children born in Scotland in 1984 weighing less than 1750 g. At 4.5 years, 636 children (71%) of the original cohort were still alive and 611 (96%) of these were assessed by a team of community child health doctors. Some children were found by them to be impossible to test using the standardised tests selected for the project. This paper describes adaptations which were made to the standardised tests to render them more accessible to children with specific difficulties. The modifications made enabled 27 children who could not initially be tested to complete some or all of the assessment procedure. It is suggested that inclusion of the assessments of children who retain intact skills despite being constrained in specific areas affords a more accurate measure of long‐term developmental handicap

Genetic influences on specific versus nonspecific language impairment in 4-year-old twins

The present study addresses the distinction between specific (SLI) and nonspecific (NLI) language impairment at an etiological level by estimating the relative genetic and environmental contributions to language impairment in children with SLI and NLI. Drawing on a large longitudinal twin study, we tested a sample of 356 four-and-a-half-year-old children with low language ability and their twin partners at home on a range of language and nonverbal measures. For children whose language and nonverbal abilities were both low (NLI), genetic influence on language impairment was moderate and shared environmental influence was substantial. A similar pattern emerged for children whose language difficulties occurred in apparent isolation (SLI), although there was a trend for the genetic effects to be smaller for SLI than for NLI: Group heritability was .18 for SLI and .52 for NLI. Probandwise cross-concordances were suggestive of some genetic overlap between these two groups, but not with a subgroup of children with more severe cognitive delay