The relationship between normal variation in IQ and common childhood psychopathology: a clinical study.

The relationship between normal variation in IQ and common psychopathology was examined in a sample of 339 5- to 16-year-olds who were seen at a tertiary psychiatric clinic. The mean IQ was 9.6 points lower (95% CI 5.5 to 13.6 points lower) for conduct than for emotional disorders, with mixed disorders in between. For these common disorders, the mean IQ was 6.0 points lower (95% CI 1.6 to 10.3 points lower) for females than males. IQ variation in the normal range was inversely related to a dimensional measure of conduct problems - an association that was not attributable to social class or mediated by scholastic attainments. Other dimensional measures of psychopathology - covering emotional symptoms, developmental immaturity and relationship difficulties - were not significantly correlated with IQ. Limitations of the study are discussed in the paper.