Object ‐ The aim of this investigation was in the first place to study the relation between mental retardation and other mental disturbances. The second aim was to study the frequency of severe and mild mental retardation in an adult Swedish population and to throw some light on the socio‐medical situation of the adult mentally retarded. Methods ‐ A primary sample, stratified with respect to population density, was extracted from the population in the age group 20‐60 years, resident in Kopparberg County, Sweden, on 1 July 1977. The mildly and severely mentally retarded in this sample were identified. Enquiry was made into the presence of additional handicaps in the mentally retarded. Social conditions including alcohol consumption and the occurence of abuse and criminality were studied in the two retarded groups and a control group representing the rest of the population. The three groups were compared by rating with the Comprehensive Psychopathological Rating Scale (CPRS), by classification of any mental illness present according to the Diagnostic and Statistical Manual of Mental Disorders (DSM‐III) and by determining their intake of psychotropic drugs and anti‐epileptics. The mildly mentally retarded and the control group were also compared with respect to neuroticism and extraversion‐introversion by rating with the Eysenck Personality Inventory (EPI). Results ‐ The study revealed a prevalence of 0.27 % for severe (IQ < 53) and 0.32 % for mild (IQ 53‐73.7) mental retardation in the age group 20‐60 years. All the severely retarded, but only just over half the mildly retarded, were known to the care authority. The majority of the former were living in some form of institution, whereas this applied to only 15 % of the mildly retarded. Nineteen per cent of the severely retarded and 4 % of the mildly retarded had manifest epilepsy. Defects of movement and of hearing were most prominent among the mildly retarded, while the frequency of specific speech disturbances was greater among the severely retarded, approximately 10 % of whom had no power of verbal communication. Visual defects were recorded in about one‐third of both groups. The study showed that alcohol intake was lower among both the severely and mildly mentally retarded than among the persons in the control group and that the frequency of abuse and criminality was as high among persons of higher intelligence as among the mentally retarded. The severely retarded, particularly the men, showed a raised psychiatric morbidity as compared with the mildly retarded and the control group. Seventy‐one per cent of the severely retarded, 33 % of the mildly retarded and 23 % of the control cases received one or more psychiatric diagnoses. The increased morbidity among the severely retarded appeared to consist mainly of psychiatric manifestations aetiologically related to brain damage or cerebral dysfunction, representing a very high frequency (53 %) of chronic psycho‐organic syndromes. The mildly mentally retarded showed a higher degree of neuroticism, with i...

Chromosome analyses were performed un 52 mildly mentally retarded adults and a control group representing the non-retarded population. Chromosomal aberrations were found in 19-2% of the mentally retarded and in 1 9% of the controls. The aberrations in the retarded group consisted of trisomy 21, fragile-X, sexchromosome aberrations and balanced translocations. The Index group included a man with a fragile site Xp22.1. The aberration in the control group consisted of a karyotype with an extra marker chromosome.