The number of individuals worldwide living with dementia is expected to triple from the current 50 million to more than 150 million by 2050 (GBD 2019 Dementia Forecasting Collaborators, 2022). Evidence has accumulated in the last two decades for the role of personality traits, which are relatively enduring patterns of thoughts, feelings, and behaviors, in incident dementia risk (Aschwanden et al., 2021; Chapman et al., 2020; Graham et al., 2021; Terracciano et al., 2014, 2017, 2021; Wilson et al., 2007). The Five Factor Model (FFM; McCrae and John, 1992) organizes personality traits into five broad domains (or factors): neuroticism (the tendency to experience distress and negative emotions), extraversion (the tendency to experience positive emotions and to be energetic), openness (the tendency to be curious and unconventional), agreeableness (the tendency to be cooperative and trusting), and conscientiousness (the tendency to be responsible and organized). A recent meta-analysis revealed that higher neuroticism and lower conscientiousness are consistent predictors of higher dementia risk, across dementia types, including Alzheimer's disease (AD), dementia assessment methods, follow-up lengths, and countries (Aschwanden et al., 2021). Likewise, another recent meta-analysis indicated that higher neuroticism and lower conscientiousness increased the vulnerability to AD tau and amyloid neuropathology (Terracciano et al., 2022a). There is less robust evidence that higher extraversion, higher openness, and higher agreeableness protect against dementia risk (Aschwanden et al., 2021). This previous research has focused primarily on the five broad domains of personality. The present study extends these findings by examining the contributions of their constituent lower-order traits to incident dementia.

Personality traits are organized hierarchically, with broad FFM domains composed of narrower traits, called facets. A focus on narrower traits could provide a clearer picture of the specific personality characteristics driving the association at the domain level (Mõttus et al., 2017; Seeboth and Mõttus, 2018; Stewart et al., 2022; Vainik et al., 2019). And, indeed, there is some evidence for associations between more specific facets of personality and dementia risk. For example, the self-discipline, industriousness, and responsibility facets of conscientiousness have been found to be protective against dementia (Sutin et al., 2018), the depression, anxiety, and angry-hostility facets of neuroticism have been associated with a higher risk of AD (Terracciano et al., 2014; Wilson et al., 2011), and the ideas facet of openness and the warmth facet of extraversion have been related to lower risk of cognitive impairment (Terracciano et al., 2022b). Facets often have more predictive power than their corresponding domain for various outcomes (Vainik et al., 2019), but there is no support yet for this hypothesis with regard to cognitive impairment (Terracciano et al., 2022b).

Facets are not the lowest level of the personality hierarchy because they can be split into more specific, narrower personality characteristics called nuances that are usually operationalized by individual questionnaire items (McCrae, 2015; Mõttus et al., 2017, 2019). Nuances are unique units of personality assessment, displaying trait-like properties of cross-rater agreement, rank-order stability, and heritability (Mõttus et al., 2017, 2019). While much existing research has focused on the five broad domains, many specific nuances are likely to drive their associations with complex outcomes and therefore provide additional information about them (Seeboth and Mõttus, 2018; Stewart et al., 2022). Indeed, nuances have been found to predict health outcomes more accurately than domains (Mõttus et al., 2017; Seeboth and Mõttus, 2018; Stewart et al., 2022). However, the vast majority of research on incident dementia has focused on either personality domains or facets, and no research has yet examined the possible contribution of personality nuances.

Based on two large longitudinal samples of older adults from the United States (US) and England, the present study examined the association between personality nuances and incident dementia. These two samples were included to test the generalizability and replicability of the association between personality nuances and dementia across samples that differ in measurement of dementia, follow-up interval, and cultural and socioeconomic characteristics. Building upon existing research (McCrae, 2015; Mõttus et al., 2017, 2019), nuances were operationalized by individual personality questionnaire items. Personality measures were harmonized across the two samples, which facilitates evaluating the replicability of the association between nuances and dementia. In line with findings for the broad FFM domains (Aschwanden et al., 2021), it was expected that items assessing higher neuroticism and lower conscientiousness would be associated with a higher risk of incident dementia, but that some specific items may drive these associations and hence reveal their finer-grained details. However, no a priori hypotheses were formulated for the specific items. Similar to polygenic scores that estimate the effects of multiple genetic variants on outcomes (Plomin and Von Stumm, 2018), this study also examined poly-item scores that aggregated the effects of personality items and risk of incident dementia in the two samples. In line with existing research (Seeboth and Mõttus, 2018; Stewart et al., 2022), these poly-item scores were hypothesized to have stronger predictive power than personality domains for incident dementia. Additional analyses were conducted to test whether clinical, behavioral, psychological and genetic risk factors for dementia accounted for the association between personality nuances and incident dementia. For comparison, the association between the five broad domains and incident dementia was also examined in the two samples.