The Twins Early Development Study (TEDS) represents Robert Plomin's major current research effort, funded as a programme grant by the UK Medical Research Council. TEDS uses the twin method comparing identical and non-identical twins to investigate genetic and environmental influences on the early development of the three most common psychological problems in childhood: Communication disorders, mild mental impairment and behaviour problems.

The twins were identified from birth records of twins born in the UK in 1994-96.  More than 15,000 pairs of twins have been enrolled in TEDS and the participating families are representative of the UK. The TEDS twins have been assessed longitudinally at 2, 3, 4 and 7 years of age in order to investigate genetic and environmental contributions to change and continuity in language and cognitive development; it is multivariate in order to examine the origins of co-morbidity; and it uses a large sample in order to study abnormal development in the context of normal development. DNA has been obtained from 5000 pairs and is being used initially in molecular genetic studies of language problems and hyperactivity.

One reason for its size is to enable TEDS to investigate the genetic and environmental aetiology of abnormal development in the context of normal development. A related goal is to study developmental problems in a representative community sample rather than studying children who were identified clinically because the latter are unlikely to be representative of children with such problems.
Another reason why a large sample size is needed is that TEDS was also designed to foster collaborative research in a new interdisciplinary research centre called the Social, Genetic and Developmental Psychiatry (SGDP) Centre whose goal is to bring together genetic and environmental influences in the study of behavioural problems of development.