Several psychiatric conditions have been associated with PWS. As noted in Chapter 10, increased risk rates have been proposed for psychotic illness and obsessive—compulsive disorder (OCD). This chapter is concerned with the latter. Obsessive—compulsive behaviour has long been recognised as part of the PWS phenotype and it is generally agreed that people with PWS have ‘obsessive personalities’, but this is not the same as OCD.

While some reports on the obsessive—compulsive behaviour in PWS have raised the possibility of heightened rates of OCD in this population, the difficulties of psychiatric diagnosis in people with learning disabilities (LD) seem to have discouraged attempts at quantification in PWS, although there are several estimates of prevalence in undifferentiated groups of people with LD. Exceptionally, the 1996 paper by Dykens, Leckman & Cassidy does seek to elucidate the nature of the obsessive—compulsive behaviour by people with PWS. We describe this paper in detail since, as well as being the most thorough investigation of obsessive—compulsive behaviour in PWS of the last decade, it raises several interesting issues.

In DSM-IV a diagnosis of OCD presumes the presence of either obsessions (intrusive, recurrent thoughts or images that exceed real-life worries) or compulsions (repetitive behaviours that a person is driven to perform in response to an obsessional thought or set of rigid rules).