Recent research has identified trial-to-trial variability in response time (RT) latencies, termed RT inconsistency, an important behavioral indicator of normal cognitive function, cognitive aging, and as a potential early indicator of cognitive pathology, and brain and central nervous system (CNS) dysfunction. Empirical and theoretical work has suggested that RT inconsistency may reflect relatively innocuous fluctuations in cognitive processing efficiency, as well as more deleterious compromises in brain and CNS function, and that RT inconsistency is susceptible to state-like and trait-like psychosocial- (increased stress/distress) and health- (increased pain, chronic health/disease conditions) related influences. However, little research has systematically disambiguated state-like and trait-like variation in RT inconsistency, or directly examined associations with psychosocial- and health-related risk factors for increased RT inconsistency in old age. These issues represent critical challenges for understanding the broad utility of RT inconsistency as a behavioral indicator of brain health, and the potentially-modifiable psychosocial- and health-related forces that influence cognitive aging. Our project draws on data from three measurement burst studies of RT-based cognitive aging (ages 65 and older) for investigating psychosocial and health-related influences on state-like and trait- like variation in RT inconsistency, and the association between RT inconsistency and cognitive function during old age. This study will address three aims: Aim 1 examines the psychosocial- (i.e., stress, affect) and health- (i.e., pain, vascular function, pulmonary function)-related influences on RT inconsistency within-persons over time and between-persons; Aim 2 examines the influence of psychosocial and health-related factors on aging- related changes in RT inconsistency; and Aim 3 examines between- and within-person variation in RT inconsistency and the use of different quantifications of RT inconsistency. The significance of this work lies in the improved understanding of: 1.) potentially modifiable psychosocial- and health-related risk factors for increased RT inconsistency and cognitive aging; and 2.) RT inconsistency as a brief, low cost and sensitive behavioral marker of both state- and trait-like fluctuations in cognitive and brain function. The innovation of this work includes: 1.) moving beyond traditional group/individual difference approaches to explicitly consider RT inconsistency as a labile construct influenced by psychosocial- and health-related factors; and 2.) the coordinated analysis of our aims across three datasets to provide a rigorous examination of the reliability and generalizability of results, and to provide a consensus on the operational definition of RT inconsistency. The impact of this work derives from improving our understanding of the modifiable psychosocial- and health- related influences that may have immediate and cumulative effects on cognitive health, and the identification of a brief, low-cost, behavioral marker (RT inconsistency) of cognitive health sensitive to state- and trait-like influences with potential utility in clinical and basic research settings.