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IQ and mental disorder in young men

Department of Health Psychology, Institute of Public Health, University of Copenhagen

HOLGER JELLING SØRENSEN, MD, PhD

Department of Psychiatry (Amager), Copenhagen University Hospital, Copenhagen

HANS HENRIK JENSEN, MagArt

Department of Health Psychology, Institute of Public Health, University of Copenhagen, Copenhagen, Denmark

JUNE MACHOVER REINISCH, PhD

Kinsey Institute for Research in Sex, Gender and Reproduction, Indiana University, Bloomington, Indiana

SARNOFF A. MEDNICK, DrMedSci

Social Science Research Institute and Department of Psychology, University of Southern California, Los Angeles, California, USA

Correspondence: Erik Lykke Mortensen, Department of Health Psychology, University of Copenhagen, Øster Farimagsgade 5A, PO Box 2099, 1014 Copenhagen K, Denmark. Tel: +45 3532 7839; fax: +45 3532 7748; e-mail: e.l.mortensen{at}pubhealth.ku.dk

Declaration of interest None. Funding detailed in Acknowledgements

Background Most research investigating the relationship between IQ and riskof mental disorder has focused on schizophrenia.

Aims To illuminate the relationship between IQ test scores in early adulthood and various mental disorders.

Method For 3289 men from the Copenhagen Perinatal Cohort, military IQ test scores and information on psychiatric hospitalisation were available. We identified 350 men in the Danish Psychiatric Central Register, and compared the mean IQ test scores of nine diagnostic categories withthe mean scores of 2939 unregistered cohort controls.

Results Schizophrenia and related disorders, other psychotic disorders, adjustment, personality, alcohol and substance-use-related disorders were significantly associated with low IQ scores, but this association remained significant for the four non-psychotic disorders only when adjusting for comorbid diagnoses. For most diagnostic categories, test scores were positively associated with the length of the interval between testing and first admission. ICD mood disorders as well as neuroses and related disorders were not significantly associated with low IQ scores.

Conclusions Low IQ may be a consequence of mental disease or a causal factor in psychotic and non-psychotic disorders.

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