The first modern use of the term ‘behavioural phenotype’ was by Professor Bill Nyhan in his 1972 paper on the challenging behaviours manifested by individuals with Lesch–Nyhan syndrome (Hall et al, 2001), and Cornelia de Lange syndrome (Berney et al, 1999). Nyhan, with the aid of cine film footage, argued persuasively that the characteristic behaviours of children with these genetic anomalies were sufficiently consistent, irrespective of the children's social backgrounds, to suggest that they were primarily attributable to the underlying genetic anomaly (Nyhan, 1972). It is now surprising to reflect on just how unacceptable such a proposal was at the time. There are a number of reasons for this. First, there was a high degree of scepticism regarding the possibility of genetically predetermined behavioural and developmental profiles, attributable to concerns regarding earlier eugenics philosophies, coupled with the rise of sociopolitical movements and a focus on the psychosocial determinants of development and behaviour. Second, the definitive Isle of Wight epidemiological neuropsychiatric study of children (Rutter et al, 1970) concluded that there were just two factors influencing the nature and severity of psychological dysfunction and challenging behaviour: the degree of intellectual disability; and the quality of the social environment of upbringing, social relationships and social experiences. These influences are not in dispute. Indeed, Emerson and colleagues, in a series of publications, reported their findings that social factors are just as important for individuals with developmental disabilities as for the general population in determining physical as well as mental health (Emerson et al, 2007). Furthermore, they identified that the cumulative risk from exposure to social disadvantage is associated with increased prevalence of psychopathology, primarily autism spectrum conditions, hyperkinesis and conduct disorders (Emerson & Hatton, 2007). They concluded that socioeconomic disadvantage may account for a significant proportion of the poorer physical and mental health of children and adolescents with intellectual disabilities. However, at the time of the Isle of Wight study, the aetiology (cause) of an individual's disorder of intellectual development was not seen as determining the nature and severity of the associated developmental, temperamental, emotional and behavioural challenges. This may have been because of limited understanding of biological, and indeed psychosocial, causality in developmental disabilities.