This chapter focuses on the psychiatric aspects of intellectual disability. Emerging research in this area has progressed from epidemiological studies to investigations into the causation of problem behaviours and comorbid mental disorders. The chapter describes the causation and the heterogeneity of dual disability. The presence of mild intellectual disability rarely has a single identifiable cause but is a consequence of both polygenic and social/environmental influences. The chapter deals with psychiatric and behaviour disorders in people with intellectual disabilities. In the case of both the presence of an additional mental disorder or of behaviour problems, a combination of developmental, biological, social or psychological factors interact and influence the occurrence of such behaviours. The chapter presents the prevention and detection of mental ill health and its treatment. It discusses the maintenance of mental health and prevention of mental ill health.

Alcohol-use disorders are among the most prevalent mental disorders in the general community. The Epidemiologic Catchment Area (ECA) study involved personal interviews with 20,000 Americans in five states. In the ECA study individuals with alcohol-use disorders were at high risk of suffering from other mental disorders. The National Comorbidity Survey (NCS) is a population survey that was undertaken between 1990 and 1992 to examine the extent of comorbidity between substance-use and nonsubstance-use disorders in the USA. A public health approach adopts a broader explanation of the causes of alcohol-related health problems. Alcohol-use disorders complicated by other comorbid mental disorders have been recognized as having a poorer prognosis and being more difficult to treat than those without comorbid disorders. Public education about the risks of alcohol use may be the best way of preventing and ameliorating the public health impact of the prevalent, milder forms of alcohol disorders.