High heritability for language delay in infancy

The birth-cohort of all twins born in England and Wales in 1994 was used for a quantitative genetic analysis of language delay and nonverbal cognitive delay. From 3,355 pairs of same-sex 2-year-old twins, individuals forming the lowest 5% of the language and lowest 5% of the nonverbal cognitive development distributions were selected to form two groups of probands. For these individuals and their co-twins, DeFries-Fulker extremes analysis was used to estimate the group heritabilities of these traits. Language delay probands showed high group heritability (73%) with a relatively low effect of shared family environment (18%). Repeating the analysis with only specific language delay probands (i.e., not comorbid with non-verbal cognitive delay) also produced a high group heritability estimate (77%). In contrast, specific nonverbal cognitive delay showed only modest genetic effects (group heritability of 22%). The group heritability estimate of language delay was significantly greater than the individual differences estimate of heritability for language ability derived from the entire sample (73% vs. 25%). This suggests that the genes for language delay may differ from the genes for normal language development. For nonverbal cognitive development, the magnitude of genetic and environmental effects appeared constant across the entire range of individual differences. Quantitative genetic results such as these should prove valuable for molecular genetics, by guiding the search for genes for complex behaviours. In the case of early language delay, identifying early-emerging genes might help to uncover factors responsible for the persistence of, or recovery from, language impairment. Funded by the UK Medical Research Council.