The psychiatrist plays a pivotal role in multidisciplinary teams that provide support and services for adults with disorders of intellectual development (DID). More than simply a historical artefact, however, this undoubtedly reflects the significance of the behavioural, medical and neuropsychiatric ‘complications’ of DID in terms of their prevalence and complexity, as well as the dynamics of families and support systems. These points notwithstanding, the psychiatrist is only one member of the multidisciplinary team, and the complexity inherent in ‘dual diagnosis’, alongside the important contributions made by relational and environmental factors, demands a range of skills, as will become apparent during the course of this chapter.

The aim of this chapter is to provide an overview of the comorbidity of DID and additional psychiatric illness. The wider concept of ‘mental health problems’ includes the large category of ‘behaviours which are experienced as challenging’, which are the subject of Chapter 5, while the pharmacological treatment of psychiatric illness in this population is discussed by Deb in Chapter 7. Therefore, the present chapter will focus on epidemiology, aetiology, assessment and diagnosis, and will give some overriding principles to consider in designing a management plan.

Epidemiology of psychiatric disorder

Although estimates of the prevalence of psychiatric disorder among individuals with DID vary widely, as a general rule it seems to be higher than in the general population. The variation in prevalence figures reflects bias in ascertainment, the diagnostic methods used and the characteristics of the populations studied, including the sample size (Smiley, 2005). Moreover, the prevalence in institutionalised samples tends to be higher. The most robust studies in epidemiological terms are summarised in Table 3.1. Even within this small number of studies, different methodologies have been employed and the prevalence figures vary widely (e.g. as low as 10% in California in the 1990 study of Borthwick-Duffy & Eyman, and as high as 31.7% in the 2008 study by Morgan and colleagues in Perth, Australia). It should be pointed out that some studies have calculated point prevalence (the studies by Rutter et al, Lund and Cooper et al), while, by their very methodology, others must be considered estimates of lifetime prevalence (the study by Morgan et al and both US studies).