The concept of ‘behavioural phenotypes in organic genetic diseases’ was first proposed by Nyhan in his Presidential lecture to the American Paediatric Association in 1971. He was referring to the fact that severe self-injurious behaviour was as much a part of the phenotype of the X-linked syndrome that subsequently was named the Lesch—Nyhan syndrome, as was any physical abnormality. In the last few years the association between specific genetically determined developmental syndromes and particular cognitive, linguistic and behavioural profiles has been increasingly recognised. This has partly been due to advances in genetic techniques, particularly in the identification of novel genetic mechanisms that give rise to developmental disorders and can account for phenotypic variation within syndromes. The best example of the latter is the relevance of the CGG triplet repeat sequence length in people with Fragile X syndrome.

In the case of genetic syndromes associated with a ‘behavioural phenotype’, it is proposed that single genes may have specific effects on brain development and function, and thereby give rise to particular patterns of social, emotional and/or cognitive development, and an increased propensity to aberrant behaviours that are recognised as being specifically, but not exclusively, associated with that syndrome. This association between genetic syndromes and ‘behavioural phenotypes’ is in contrast to the usual model of behavioural ‘normality’, which presumes an interaction between the inheritance of allelic variants of several genes and shared or non-shared environmental influences on cognitive development, and on behaviour patterns and personality.