Autism spectrum disorder (ASD) is one of the leading causes of lifetime developmental disability (Fombonne, 2003). The diagnosis includes childhood autism, atypical autism and Asperger syndrome; these are often alternatively grouped as pervasive developmental disorders. DSM- 5 has integrated these separate disorders into a single ‘autism spectrum disorder’ diagnosis (American Psychiatric Association, 2013). They are recognised as complex neurodevelopmental disorders, often becoming clinically apparent in the second to third year of life. The DSM-5 diagnosis is based on disturbance in two domains: persistent deficits in social communication and social interaction across multiple contexts; and restricted, repetitive patterns of behaviour, interests or activities. Up to two-thirds of affected individuals will present with a degree of global intellectual disability, although some may have a very uneven profile of abilities. Accurate diagnosis, usually made by a combination of direct observation of behaviour and informant history, is complicated by considerable heterogeneity in the manifestation of these core deficits, by variation in ability level and by developmental changes. However, it is clear that ASD persists across the life span, and produces varied and complex needs in adult life (Lord et al, 2001). The course of development into old age is, as yet, largely unknown.

Prevalence figures vary widely, depending on the definition of ‘caseness’ and assessment tools used. UK studies have reported a prevalence of around 1.2% (Baird et al, 2006). There are no accurate prevalence figures available for adults but in 2011 it was estimated there were 5.3 million adults with diagnosed ASD across Europe, the USA and Japan (Nightingale, 2012). A male excess of between 3:1 and 4:1 is generally observed (Fombonne, 2003). ASD is associated with an estimated annual UK cost in childhood of £3.1 billion (Buescher et al, 2014), greater than asthma, diabetes or other intellectual disability.

Maladaptive behaviours and comorbid psychiatric symptoms are common in individuals with ASD and are strongly associated with carer stress (Lecavalier et al, 2006).

Autism spectrum disorder is thus a relatively common, chronic, potentially substantially disabling disorder, with significant costs both to the affected individual and to family members. There are no established definitive treatments for the core social impairment.