Associations have been described between lower IQ and serious mental illness. Associations between common mental disorders (CMDs) and IQ have received little research. The objective of this study was to investigate the association between verbal IQ and CMD symptoms and diagnoses, and to investigate the role of potential mediating and confounding factors.

Data were analysed from a British national survey with an analysed sample of 8054 people aged 16–74 years. Associations between verbal IQ (NART) and mental symptoms/disorders (CIS-R) were analysed with covariates including education, social class, income, debt, problem drinking, life events, physical health and relationship quality.

CMD was associated with lower IQ. This association was stronger for depressive disorder/symptoms than for generalised anxiety disorder/symptoms. The most important covariates were education, social class, income and relationship quality.

The association between lower IQ and CMD is partly accounted for by adverse social/socioeconomic conditions. Stronger associations for depression than anxiety may indicate an effect of IQ on the way mental distress is communicated.

Only a minority of trauma-exposed individuals go on to develop post traumatic stress disorder (PTSD). Previous studies in high-income countries suggest that maladaptive family functioning adversities (MFFA) in childhood may partially ex-plain individual variation in vulnerability to PTSD following trauma. We test in a lower middle income setting (Sri Lanka) whether: (1) MFFA moderates the association between exposure to trauma and later (a) PTSD (b) other psychiatric diagnoses; (2) any moderation by MFFA is explained by experiences of interpersonal violence, cumulative trauma exposure or other psychopathology.

We conducted a population study of 3995 twins and 2019 singletons residing in Colombo, Sri Lanka. Participants completed the composite international diagnostic interview, including nine traumatic exposures and a questionnaire on MFFA.

In total, 23.4% of participants reported exposure to MFFA. We found that (1) MFFA moderates the association between trauma exposure and both (a) PTSD and (b) non-PTSD diagnosis. (2) This was not explained by interpersonal violence, cumulative trauma exposure or other psychopathology.

In our sample MFFA moderates the association between trauma and PTSD, and the association between trauma and non-PTSD psychopathology.