The primary purpose of this dissertation was to examine age-related trajectories in illness behavior—or perceptions, evaluations, and responses to symptoms—across the late-life transition, the predictive role of perceived social support availability, and the extent to which illness behavior mediates the association of social support with subsequent functional decline. For Study 1, two large population-based samples were drawn from the Swedish Adoption/Twin Study of Aging (SATSA) and the Sex Differences in Health and Aging study (GENDER) to examine longitudinal measurement invariance in illness behavior. The extent to which social support availability from friends versus family members predicted reduced levels and change in illness behavior across 17 years from mid- to late-adulthood in SATSA was also examined. Confirmatory factor analyses supported strict factorial invariance in the illness behavior factor across four waves in SATSA, whereas partial, weak factorial invariance was supported in GENDER. Latent growth models suggested, a small, linear increase in illness behavior across age, and perceived support from friends and family both predicted reduced levels of illness behavior—but not change—after controlling for sex, comorbidity, SES, marital status, and age at study entry.

For Study 2, age-related change in functional difficulty was evaluated across 23 years in SATSA, as well as the extent to which social support availability (again, from friends versus family) buffered decline. Additionally, longitudinal mediation models evaluated the extent to which illness behavior explained the association between social support availability and subsequent functional status and change. Overall, findings suggested piecewise growth in functional difficulty, with stability prior to age 75 and a linear increase afterward. Higher family support availability predicted faster decline, whereas friend support was not associated. Furthermore, illness behavior status mediated the association between social support availability and functional status; whereas intra-individual growth in illness behavior separately predicted a faster rate of functional decline. Collectively, these results suggest that illness behavior, as a unifying individual difference construct, exhibits systematic intra-individual change and significant variability during the transition to late adulthood, and represents a potentially important mediating and independent pathway linking perceived social support availability with subsequent physical functioning.