`DiagnosticOvershadowing'Amongst CliniciansWorkingwithPeoplewith IntellectualDisabilitiesintheUK Jonathan Mason  and Katrina Scior y  John Howard Centre (CASS), 2 Crozier Terrace, Hackney, London E9 6AT, UK, y Subdepartment of Clinical Health Psychology, UCL, Gower St., London WC1E 6BT, UK Accepted for publication 26 October 2003 Background Mentalhealthproblemsareknowntobehard to recognize in people with intellectual disabilities. One factor that may contribute to this is the diagnostic over- shadowing bias, which describes the tendency of the clinicians to overlook symptoms of mental health pro- blemsinthisclientgroupandattributethemtobeingpart of `having an intellectual disability'. Although a small amountofresearchhasinvestigatedthisintheUSA,very little has taken place in the UK. Materials and methods Two clinical vignettes were pro- duced. Both described identical clinical problems, except that one vignette described a man with an IQ of 58 and impaired social functioning (i.e. an intellectual disability) and one a man with an IQ of 108 and normal social functioning(i.e.non-intellectuallyimpaired).Psychiatrists (n  274) and clinical psychologists (n  412) throughout UKwererandomlyassignedtoeitherthelowIQornormal IQ condition, and sent a corresponding clinical vignette. Results One hundred and thirty-three psychologists and 90psychiatristsresponded.Diagnosticovershadowingdid appeartobepresent.Cliniciansweremorelikelytorecog- nizearangeofsymptomsinthosewithIQsinthenormal range than those with intellectual disabilities. Further- more, psychiatrists appeared more likely to make this error under some circumstances than clinical psycholo- gists. Conclusions Diagnostic overshadowing may contribute to thedif®cultiesthatmentalhealthprofessionalscommonly experience in identifying mental health problems in peo- ple with intellectual disabilities. However, the methodol- ogycommonlyusedinthistypeofresearchhasanumber of weaknesses, and would bene®t from an alternative approach. Keywords: diagnostic overshadowing, intellectual disabil- ities, UK Introduction People with intellectual disabilities are known to be at an increasedriskofdevelopingmentalhealthproblems.The reasons for this vulnerability are unclear, and a range of biological, sociocultural, cognitive, systemic and psycho- dynamic explanations have been forwarded (Matson & Sevin 1994). Further, although a signi®cant amount of research has been focused on psychological therapies for mental health problems, until recently little attention has been given to their application to people with intellectual disabilities. This, in combination with a number of other factors (such as a historical trend to suppose that people with intellectual disabilities struggle to make use of psychological therapy), means that this client group has relativelylittleaccesstotherapyservices(Matson&Barrett 1982; Bender 1993; Matson & Sevin 1994). A possible additional reason for the lack of psychological therapy provided to people with intellectual disabilities is the presence of the diagnostic overshadowing bias, which describes the reluctance of mental health professionals to acknowledge mental health problems in people with intellectual disabilities, and their tendency to overlook them as being fundamentally part of the intellectual dis- ability itself (Reiss et al. 1982). This study aims to inves- tigate the relevance of this bias to clinicians working in servicesforpeoplewithintellectualdisabilitiesintheUK. The term `diagnostic overshadowing' refers speci®cally tothefactthat,inthepresenceofanintellectualdisability, accompanyingmentalhealthproblemsbecomelesssalient Journal of Applied Research in Intellectual Disabilities 2004, 17, 85±90 # 2004 BILD Publications