Mild Behavioral Impairment (MBI) has been proposed to detect neuropsychiatric symptoms (NPS) associated with dementia development, but evidence from population-based settings is limited.

To (i) investigate the association between NPS in late life and the onset of dementia over 15 years in community-dwelling older adults, and (ii) test the interplay of NPS and Cognitive Impairment, No Dementia (CIND) in dementia development.

2597 dementia-free individuals aged 60+ from a longitudinal population-based cohort underwent cognitive assessments over 15 years. Thirty clinically-assessed NPS were mapped into five domains and, within each domain, a z-score was computed from the sum of the NPS’s points. MBI was identified when the z-score was above 1.5 standard deviations (SDs) in at least one of 5 neuropsychiatric domains. Based on a cognitive battery, CIND was defined as scoring ≥1.5 SDs below age-specific means in at least one cognitive domain. Dementia was diagnosed by DSM-IV criteria following standardized procedures.

MBI, present in 16.1% of the sample, was associated with a higher hazard of incident dementia over 15 years (multi-adjusted hazard ratio [HR] 1.68, 95% confidence interval [CI] 1.31–2.17). Decreased motivation and social inappropriateness were the domains associated with incident dementia (HR 2.23, 95%CI 1.59–3.14 and HR 3.29, 95%CI 1.83–5.94, respectively). Compared to those with neither, individuals with either MBI (HR 1.37, 95%CI 1.00–1.90) or CIND (HR 2.22, 95%CI 1.73–2.84) had increased dementia incidence, especially when co-occurring (HR 4.41, 95%CI 3.04–6.39).

Late life NPS, especially with co-occurring cognitive impairment, was associated with a higher dementia incidence.