BackgroundIntelligence is the aggregate or global capacity of the individual to act purposefully, to think rationally and to deal effectively with the environment. Previous studies have shown that individuals with intellectual disability, IQ < 70, have increased risk of being diagnosed with one or more mental disorders. We wanted to investigate if this also applies to individuals with IQ between 70 and 85.MethodsIn this study, data was abstracted from a longitudinal follow-up study of individuals with low birth weight and a control group. In the present study, mental health of participants with borderline IQ, defined as a full IQ score 70–84, were compared with mental health of a reference group with full IQ scores ≥85. Mental health at age 19 was assessed using the Schedule for Affective Disorder and Schizophrenia for School-age Children Present and Lifetime (K-SADS P/L) whereby scores meeting the diagnostic criteria for a mental disorder were defined as having mental health problems. In addition the participants completed the ADHD-rating scale and the Autism Spectrum Quotient form (AQ). Logistic regression analyses were used to calculate odds ratio (OR) with 95% confidence intervals (CI) for high scores on the K-SADS.ResultsThirty participants with borderline IQ and 146 controls were included. Sixteen (53%) of the participants with borderline IQ met the diagnostic criteria on the K-SADS for any diagnosis compared with 18 (12%) in the reference group (OR: 6.2; CI: 2.6–14.9). In particular the participants with borderline IQ had excess risk of ADHD and anxiety. These associations were slightly attenuated when adjusted for birth weight and parents’ socioeconomic status.Conclusions53% of the participants with borderline IQ had increased risk for a research assessed psychiatric diagnosis compared to about one in ten in the reference group. The group with borderline IQ also had higher total scores and higher scores on some sub-scores included in the Autism Spectrum Quotient form. Our results points towards an increased vulnerability for mental illness in individuals with borderline low IQ.Trial registrationThe main study is recorded by the Regional Committee for Health Research Ethics in Mid-Norway (as project number 4.2005.2605).