Capturing Transitions and Trajectories: The Role of Socioeconomic Status in Later Life Disability

CAN transitions and trajectories be simultaneously examined to better capture the life course process of disability in later life? If so, what is the added benefit in testing the effects of known predictors over time? Do multiple components of socioeconomic status (education and financial resources) differ in their effects on disability onset and accumulation in later life? Disability is primarily conceptualized using a life course framework (Verbrugge & Jette, 1994) with an emphasis on individual transitions and trajectories. Timing is a key component of life course transitions and active life expectancy (Crimmins, Saito, & Ingegneri, 1997). Transitions are embedded in trajectories, giving shape and meaning to the trajectory as a whole. The study of trajectories in life course research is primary in investigating the management of demands and resources over time (Halfon & Hochstein, 2002). Although transitions and trajectories are interdependent facets of life course processes, they are rarely studied simultaneously. The intersection of life course and health research suggests that key predictors of health work differently on transitions and trajectories (Kuh, Ben-Shlomo, Lynch, Hallqvist, & Power, 2003). Measurement of disability has focused increasingly on its dynamic nature, and analytic strategies capture either transitions or, more recently, individual level trajectories (see Manton, 1988; Taylor & Lynch, 2004). However, no study to the knowledge of the author has addressed both components simultaneously in measuring socioeconomic predictors over time. Given the observed declines in disability prevalence among older adults in the United States (Freedman, Schoeni, Martin, & Cornman, 2007), researchers have increasingly focused on underlying factors, and particularly education (Freedman & Martin, 1999). The connection of individual-level socioeconomic status and health is central to social stratification and health research across the life course (House, Lantz, & Herd, 2005), particularly the disentangling of education and income effects on the onset and progression of poor health. Research suggests that education works to prevent or delay poor health, whereas income has both a preventive role and a “muting” effect on severity once poor health has occurred. However, the findings for disability outcomes in later life over time are inconsistent (see Grundy & Glaser, 2000; Maddox & Clark, 1992). In this article, I bridge theories of the disablement process and the social stratification of health within the life course framework to disentangle the effects of socioeconomic status on disability in later life. I test these effects on the first transition (onset) and subsequent trajectory (progression) of disability over time, conceptualizing and measuring disability as a life course process. The analyses presented in this study serve to disentangle these effects as the first step in understanding how socioeconomic status works in the causal pathways leading to differential trajectories of disability in later life with regard to transitions and trajectories. In addition, they serve as an example of a novel approach to trajectory analyses where timing of transition is tested separately from progression of poor health, therefore outlining a more nuanced analysis of onset, severity, and recovery of other health outcomes over time in future research.