Longitudinal change in spousal similarities in mental health: Between-couple and within-couple perspectives

Research based on between-couple perspectives indicate that spouses share similarities in a range of psychosocial characteristics. In this study, the authors add to existing research by examining spousal similarities in mental health and its time-related change from both between-couple and within-couple perspectives. The authors apply latent growth models to 9-wave annual longitudinal data obtained from 3,410 adult couples in the Household, Income and Labor Dynamics in Australia Survey (HILDA; M_{age wives} = 48 years, M_{age husbands} = 50 years). In a first step, the authors corroborate extant findings from a between-couple perspective that spouses show considerable similarities in levels of and changes in mental health. In a second step, they calculate a within-couple similarity index (i.e., using absolute difference scores calculated based on the 2 partners' mental health). The authors show that mental health ratings between partners within a given spousal unit differed considerably (0.88 SD) and that these differences remained relatively stable over time. Examining between-couple differences in within-couple similarity revealed that larger discrepancies were associated with lower mental health (of individual partners), chronic health conditions, less marital satisfaction, and elevated risks for dissolution of the partnership. The authors discuss ways to integrate this counterintuitive set of findings with research originating from between-couple and within-couple perspectives, argue that a certain degree of spousal differences-if kept within certain bounds-can be adaptive serving developmental and relationship functions, and suggest routes for future inquiry on spousal similarities.