Correlations between socioeconomic status, IQ and aetiology in mental retardation: a population-based study of Norwegian children.

Abstract

Mental retardation (MR) (IQ< or = 70) is one of the most frequent and debilitating neurological handicaps in children. The aetiologies of MR are multiple and to a certain extent related to social class and the degree of MR. However, recent epidemiological data assessing these relationships are scarce. The objective of the present study was therefore to estimate the associations between socioeconomic status (SES), level of IQ, and causative factors in children with MR. The investigation was designed as a cross-sectional population-based study of Akershus County, outside Norway's capital Oslo. The source population comprised 30 037 children born between 1980 and 1985. Of 178 children aged 8-13 years, 79 had severe MR (SMR) (IQ<50) and 99 had mild MR (MMR) (IQ 50-70). SES was grouped from I to V according to parental education. Diagnostic work-up regarding aetiology and medical diagnoses was extensive. The aetiology of MR was divided into two main groups: biopathological MR (n = 143), and unspecified MR, meaning MR of unknown aetiology (n = 35). Biopathological MR was further subdivided into four sub-groups: prenatal, perinatal, postnatal and undetermined timing of the damaging event. SES III, IV and V (the lower grades) increased the risk of MMR. In SES IV and V there was an increased risk of unspecified MR, odds ratio 7.0 and 5.6 (95% CI 1.3, 39.0 and 1.1, 30.0) respectively, compared to SES I. IQ in unspecified MR was significantly higher than in the biopathological aetiology sub-groups. The study showed that parents of children with SMR had a higher socioeconomic level than parents of children with MMR. The findings were also consistent with a partial overlap between unspecified MR and the lower end of normal IQ distribution in the general population.