The classic genetic approaches of family, twin and adoption studies have provided considerable evidence that genetic influences play an important role in the development of child behaviour and cognition. There have also been remarkable advances in the application of molecular methods in medicine, together with technological progress in mapping and sequencing the human genome. As a result, many are now persuaded that the time is right to focus on the identification of genes that may give rise to childhood psychiatric and behavioural disorders. Expectations aroused by work in this area are high, so that molecular genetic strategies now have a pivotal role in research aimed at elucidating both the biological basis (nature) and environmental basis (nurture) of childhood behavioural and neurodevelopmental disorders, with the hope of major developments in prevention and treatment. In this review we focus on contemporary approaches to gene discovery in complex disorders and describe how they have been applied in child psychiatry and psychology. COMBINING QUANTITATIVE GENETIC AND MOLECULAR GENETIC APPROACHES Molecular genetic research in the field of child development and psychopathology is comparatively new. In part, this is because of difficulties in defining childhood disorders and the scarcity of sufficiently detailed family, twin and adoption studies demonstrating familial resemblance between close family members and the relative contribution of genes and environment. In many clinical areas the question of familial risk is only now starting to be addressed using more rigorous protocols. Indeed, the relative ease with which molecular genetic studies can be performed has in many cases led to molecular approaches being pursued in advance of good phenotypic definition. As a result many groups are developing both quantitative and molecular approaches side by side, since they are complementary in the investigation of complex behavioural disorders.

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