© Cambridge University Press, 1993 This article was originally published in The Journal of Child Psychology and Psychiatry and Allied Disciplines, Vol. 34, No. 8, pp. 1327-1350, November 1993. It is provided to Asperger Syndrome Coalition of the United States, Inc. for use on its website with the express written permission of Cambridge University Press. It may be downloaded and printed for personal reference, but not otherwise copied, altered in any way or transmitted to others (unless explicitly stated otherwise) without the written permission of Cambridge University Press. Hypertext links to other Web locations do not constitute any endorsement or authorization by Cambridge University Press.

Abstract:

This paper describes a total population study of Asperger syndrome using a two-stage procedure. All school children in an outer G–teborg borough were screened. Final case selection based on clinical work-up showed a minimum prevalence of 3.6 per 1,000 children (7-16 years of age) using Gillberg and Gillberg's criteria and a male to female ratio of 4:1. Including suspected and possible Asperger's syndrome cases, the prevalence rose to 7.1 per 1,000 children and the male:female ratio dropped to 2.3:1. These findings are discussed as they relate to previously published results in the field and to findings obtained using Szatmari et al.'s and ICD-10 draft criteria for the disorder.

Introduction

Since Lorna Wing's vivid clinical account of "Asperger's syndrome" appeared in 1981 (Wing, 1981), about 50 articles have been published on the subject. However, the epidemiology of Asperger syndrome (AS) has never been specifically studied. Considering that the 10th revision of the International Classification of Diseases (ICD-10) (World Health Organization, 1992) includes "Asperger's syndrome" as a subcategory of Pervasive Developmental Disorders, obtaining reliable prevalence estimates becomes mandatory. A review of indirect data from neighboring fields revealed that a minimum prevalence in children who attend normal schools would be 2.6 per 1,000 children, i.e. several times higher than that reported for autism (Gillberg & Gillberg, 1989). The rate among mentally retarded children appears to be similar (Gillberg, Persson, Grufman & Themnr, 1986). Still, given the relatively low prevalence of mental retardation, AS cases recruited from this subpopulation add little to the total population prevalence. The available data originate from surveys not particularly geared to examining AS prevalence. The object of the present study was to provide reliable epidemiological data on AS.

Defining AS

The definition of a "case" presents difficulties in all prevalence studies on childhood psychiatric disorders (Schwartz Gould, Wunsch-Hitzig & Dohrenwend, 1981), and AS is no exception. The main reason for this is that we lack a true "gold standard" (Szatmari, 1989). Furthermore, there is no universal agreement on diagnostic characteristics (Szatmari, 1991). Asperger's own descriptions are penetrating but not sufficiently systematic (Asperger, 1944). His frame of reference was Bleuler's typology (i.e. "autistic psychopathy") that is out of keeping with current diagnostic manuals. Wing, in her pioneer paper, did not explicitly spell out which symptoms had to be present for a diagnosis to be made, though more specified characteristics are presented in a recently published paper (Wing, 1991). Other authors, such as Gillberg and Gillberg (1989), and Szatmari, Brenner and Nagy (1989) from Asperger's and Wing's work, clinical experience and comparative studies, have proposed operationalized diagnostic criteria. In several other publications (Bosch, 1970; Van Krevelen, 1971; Wolff & Barlow, 1979; Wolff & Chick, 1980; Nagy & Szatmari, 1986; Kerbeshian & Burd, 1986; Rutter & Schopler, 1987; Kay & Kolvin, 1987; Bowman, 1988; Frith, 1989; Tantam, 1988a, b, 1991; Goodman, 1989; Bishop, 1989; Baron-Cohen, 1988; Kerbeshian, Burd & Fisher, 1990; Green, 1990; Howlin, 1991; Cox, 1991; Wolff, 1991a, b; Wolff, Townsend, McGuire & Weeks, 1991; Gillberg, 1992) the delineation of the syndrome vis-ý-vis autism, schizoid personality and schizotypal personality disorder, Tourette syndrome, semantic-pragmatic language disorder and obsessive-compulsive disorder has been discussed. The criteria of the Gillbergs (1989, 1991) and Szatmari et al. (1989) have been elaborated with the purpose of making them compatible with current diagnostic manuals. The proposals of these two groups have much in common, but differ in that the Gillbergs underline the children's obsessional and narrow pattern of interest and Szatmari et al. highlight their social isolation. In short, Szatmari et al.'s criteria appear to be slightly more in line with "the passive" and the Gillbergs' criteria with "the active but odd" in Wing's typology of autism spectrum disorders (Wing & Gould, 1979; Wing & Attwood, 1987). The ICD-10 draft criteria for AS (WHO, 1990) are similar to those of the Gillbergs, but with one important exception. In the ICD-10 "the term Asperger's syndrome proposes that there is a group of individuals who have a disorder of social development similar to that found in infantile autism, but with a pattern of early language development that appears grossly normal" (Cox, 1991). Accordingly, criteria of abnormalities in verbal communication are not included. However, by not including operationalised criteria of abnormalities, such as odd prosody and semantic-pragmatic problems, the ICD-10 criteria exclude features that other authors in the field find important and characteristic of AS (Asperger, 1944; Wing, 1981, 1991). Current diagnostic criteria for AS are outlined in Tables 1-3. In the present study we applied the diagnostic criteria for AS outlined by Gillberg and Gillberg (1989) and elaborated in Gillberg (1991), since, at the time of embarking on the study, these were the only published criteria available to us. Also, we report the result of applying Szatmari et al.'s (1989) and the ICD-10 (WHO, 1990) diagnostic criteria for the disorder.