Abstract
Background: There has been ongoing interest in the role of intelligence in longer-term educational and occupational achievement and social adjustment. The aims of this study were to examine the extent to which IQ in middle childhood (8-9 years) was prognostic of future outcomes when due allowance was made for confounding personal and social factors. Methods: Data were gathered on (WISC-R) IQ at ages 8-9 years and a range of educational and social adjustment measures over the course of the Christchurch Health and Development Study, a 25-year longitudinal study of a birth cohort of 1,265 New Zealand children. Results: IQ assessed at ages 8-9 years was related to a range of outcomes: later crime (offending, arrest/conviction); substance use disorders (nicotine dependence, illicit drug dependence); mental health (anxiety, suicidality); sexual adjustment (number of sexual partners, pregnancy); educational achievement (school leaving qualifications, tertiary qualifications); and occupational outcomes (unemployment, income). However, intelligence was largely unrelated to many of these outcomes: crime, mental health, sexual behaviours, and illicit substance dependence after statistical adjustment for early behaviour problems and family background. Strong relationships remained between childhood intelligence and later educational and occupational outcomes. Conclusions: Much of the association between early intelligence and later social adjustment is mediated by childhood conduct problems and family social circumstances. However, strong relationships exist between early intelligence and later academic achievement and income independently of these factors.