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Changes in behavior and personality are 1 criterion for the diagnosis of dementia. It is unclear, however, whether such changes begin before the clinical onset of the disease.To determine whether increases in neuroticism, declines in conscientiousness, and changes in other personality traits occur before the onset of mild cognitive impairment or dementia.A cohort of 2046 community-dwelling older adults who volunteered to participate in the Baltimore Longitudinal Study of Aging were included. The study examined personality and clinical assessments obtained between 1980 and July 13, 2016, from participants with no cognitive impairment at first assessment who were followed up for as long as 36 years (mean [SD], 12.05 [9.54] years). The self-report personality scales were not considered during consensus diagnostic conferences.Change in self-rated personality traits assessed in the preclinical phase of Alzheimer disease and other dementias with the Revised NEO Personality Inventory, a 240-item questionnaire that assesses 30 facets, 6 for each of the 5 major dimensions: neuroticism, extraversion, openness, agreeableness, and conscientiousness.Of the 2046 participants, 931 [45.5%] were women; mean (SD) age at first assessment was 62.56 (14.63) years. During 24 569 person-years, mild cognitive impairment was diagnosed in 104 (5.1%) individuals, and all-cause dementia was diagnosed in 255 (12.5%) participants, including 194 (9.5%) with Alzheimer disease. Multilevel modeling that accounted for age, sex, race, and educational level found significant differences on the intercept of several traits: individuals who developed dementia scored higher on neuroticism (β = 2.83; 95% CI, 1.44 to 4.22; P < .001) and lower on conscientiousness (β = −3.34; 95% CI, −4.93 to −1.75; P < .001) and extraversion (β = −1.74; 95% CI, −3.23 to −0.25; P = .02). Change in personality (ie, slope), however, was not significantly different between the nonimpaired and the Alzheimer disease groups (eg, neuroticism: β = 0.00; 95% CI, −0.08 to 0.08; P = .91; conscientiousness: β = −0.06; 95% CI, −0.16 to 0.04; P = .24). Slopes for individuals who developed mild cognitive impairment (eg, neuroticism: β = 0.00; 95% CI, −0.12 to 0.12; P = .98; conscientiousness: β = −0.09; 95% CI, −0.23 to 0.05; P = .18) and all-cause dementia (eg, neuroticism: β = 0.02; 95% CI, −0.06 to 0.10; P = .49; conscientiousness: β = −0.08; 95% CI, −0.16 to 0.00; P = .07) were also similar to those for nonimpaired participants.No evidence for preclinical change in personality before the onset of mild cognitive impairment or dementia was identified. These findings provide evidence against the reverse causality hypothesis and strengthen evidence for personality traits as a risk factor for dementia.