The Twins' Early Development Study (TEDS) is a large-scale longitudinal study of twins from early childhood through adolescence. Since its conception, TEDS has had as its focus the study of problematic development within the context of normal variation, mainly in the development of language, cognitive and academic abilities and behavior problems from multivariate quantitative and molecular genetic perspectives. TEDS twins have been assessed at 2, 3, 4, 7, 9, 10 and (currently) 12 years of age, and DNA collected from more than 12,000 children. Identified from birth records of twins born in the United Kingdom between 1994 and 1996, more than 15,000 pairs of twins originally enrolled in TEDS, and well over 13,000 pairs — representative of the UK population — remain involved in the study to date. Similar to many other twin and adoption studies, TEDS data indicate that both genetic and environmental influences are important in nearly all areas of behavioral development. Multivariate genetic analyses allow researchers to go beyond this basic nature–nurture question, and TEDS results suggest that, especially in the area of learning abilities and disabilities, genes are generalists and environments are specialists. That is, genes largely contribute to similarity in performance within and between learning abilities and disabilities and across age, whereas the environment contributes to differences in performance. Quantitative genetic findings such as these chart the course for molecular genetic research. The TEDS dataset is proving valuable in genome-wide association research that tries to identify some of the many genes responsible for the ubiquitous heritability of behavior.

Previous work suggests that most clinically significant language difficulties in children do not result from acquired brain lesions or adverse environmental experiences but from genetic factors that presumably influence early brain development. We conducted the first twin study of language delay to evaluate whether genetic and environmental factors at the lower extreme of delayed language are different from those operating in the normal range. Vocabulary at age two was assessed for more than 3000 pairs of twins. Group differences heritability for the lowest 5% of subjects was estimated as 73% in model-fitting analyses, significantly greater than the individual differences heritability for the entire sample (25%). This supports the view of early language delay as a distinct disorder. Shared environment was only a quarter as important for the language-delayed sample (18%) as for the entire sample (69%).

The current findings suggest that (i) maternal prenatal risks and fearless temperament showed a dose-response relationship with CP and CU (i.e., higher clustering of risks tended to relate to both higher levels and the co-occurrence of CU with CP), and (ii) intervention programs that aim to improve behavioural outcomes may consider targeting specific temperamental features in both boys and girls.

Parent reports have been widely used to assess child behaviours in the socio-emotional domain, but seldom have been used to assess behaviours within the cognitive domain. The present study examines the ability of parent reports and parent-administered tasks obtained through the post to assess non-verbal cognitive abilities in early childhood. In a sample of 107 2-year-olds, age-corrected scores on parent reports and parent-administered tasks assessing non-verbal reasoning significantly predicted performance on the Mental Development Index (MDI) of the Bayley Scales of Infant Development-I1 two weeks later (r = .49 and Y = .41,p < ,0001, respectively). The multiple correlation between the two components and the MDI was .55 Cp < .0001).This ability ofparental assessments to predict the MDI is comparable to the predictive power of standard tester-administered measures at this age, even though the parent measure specifically excludes verbal items that are included in the MDI. Adding parent reports o f language development significantly improved the prediction of the MDI ( R = .66, p < .0001). In addition, higher within-domain than cross-domain correlations reflect a significant ability of parents to discriminate verbal and nonverbal abilities.

The genetic and environmental etiologies of diverse aspects of language ability and disability, including articulation, phonology, grammar, vocabulary, and verbal memory, were investigated in a UK sample of 787 pairs of 4½ year-old same-sex and opposite-sex twins. Moderate genetic influence was found for all aspects of language in the normal range. A similar pattern was found at the low end of the distribution with the exception of two receptive measures. Environmental influence was mainly due to nonshared factors, unique to the individual, with little influence from shared environment for most measures. Genetic and environmental influences on language ability and disability are quantitatively and qualitatively similar for males and females. (109) 2 RUNNING HEAD: Individual language measures