Intellectual Functioning in In-Patients with Substance Use Disorders: Preliminary Results from a Clinical Mediation Study of Factors Contributing to IQ Variance

Abstract

Aims: To investigate the direct effect of different childhood difficulties on adult intelligence coefficient (IQ) and their possible indirect effect through the mediating pathways of education and severity substance use. Participants: Ninety in-patients aged 19–64. The participants had abstained from substance use for at least 6 weeks and had different substance use profiles. Measurements: Substance use disorder (SUD) and psychiatric illnesses were diagnosed according to the International Classification of Diseases 10th edition criteria. IQ was measured with the Wechsler Adult Intelligence Scale, 4th edition. Childhood difficulties, severity of substance use and level of education were assessed through a self-report questionnaire. Findings: Mean full scale IQ for the studied population was 87.3. Learning and attention deficit/hyperactivity difficulties in childhood were directly related to adult IQ. Education had a mediating effect between childhood learning difficulties/conduct problems and the verbal comprehension index. There was no significant difference in IQ due to the specific substance used or severity of substance use. Conclusion: IQ variance in in-treatment individuals with SUD was related to childhood functioning alone or through the mediator of education. Substance-related factors did not contribute to IQ variance. The results fit a normal theory of IQ development with commonly known risk factors and no disturbing effect of substance use.