Background Personality traits have been liked to cognitive outcomes such as dementia, but whether these associations are robust to the effects of third variables remains the subject of debate. We examined the role of socioeconomic status, depression (history and depressive symptoms), health behaviours and chronic conditions in the association of the big-5 personality traits with cognitive performance, cognitive impairment and incidence of dementia.

Methods Data on 6135 persons (30% women), aged 60–83 years in 2012/13, are drawn from the Whitehall II Study. Participants responded to the 26-item Midlife Development Inventory to assess personality traits (openness, conscientiousness, extraversion, agreeableness and neuroticism), underwent cognitive testing in 2012/13 and 2015/16 and were followed for incidence of dementia (N=231) until 2019.

Results Logistic regression, adjusted for sociodemographic factors, suggested a cross-sectional association with cognitive impairment for four of the five traits but only neuroticism was associated with incident cognitive impairment. All associations were completely attenuated when the analyses were adjusted for depression. Cox regression (mean follow-up: 6.18 years) adjusted for sociodemographic variables showed higher conscientiousness (HR per SD increment=0.72; 95% CI 0.65 to 0.81) and extraversion (HR=0.85; 95% CI 0.75 to 0.97) to be associated with lower dementia risk; higher neuroticism (HR=1.32; 95% CI 1.17 to 1.49) was associated with increased risk. Further adjustment for depression led to only conscientiousness retaining an association with dementia (HR=0.81; 95% CI 0.69 to 0.96), which was robust to adjustment for all covariates (HR=0.84; 95% CI 0.71 to 0.91; P=0.001).

Conclusion Our results show that only conscientiousness has an association with incidence of dementia that is not attributable to socioeconomic status or depression. The association of neuroticism with dementia was explained by depression.