Interdependence has been suggested as a defining feature of close partner relationships. More specifically, in couples there are relationships between partners’ life situations, behaviors, feelings, and characteristics. Partners share their happiness and sorrow, as well as some of their habits. Interdependence is especially evident when one partner gets ill or disabled. Such life events can cause loss of mental health and well-being not only for the affected individual, but also for the spouse. In this thesis, partner associations were investigated from two different perspectives: spousal similarity and spousal caregiving. Dyadic data from between 8,341 and 21,150 marital or cohabitating couples who participated in various waves of a population-based health screening, the North Trøndelag Health study (HUNT), were used for all analyses. Couples were identified using prospective registry information from Statistics Norway (SN). SN data also made it possible to identify future couples among the participants, getting married up to 16 years after participating in HUNT.
We aimed to explore to what extent spousal similarity is due to non-random mating, and to what extent partners converge over the course of the relationship. The first paper focused on spousal similarity in lifestyle habits (smoking, alcohol consumption, and exercise), and the second paper examined spousal similarity regarding psychological variables (mental health, life satisfaction, type A-personality, and optimism). By demonstrating significant correlations between future spouses measured on average 12 years before they entered into marriage, our results support the theory of non-random mating for all variables, except for the personality trait optimism. Significant convergence was observed for all study variables during the time before entry into marriage. During the time after entry into marriage, various patterns of convergence and divergence were predicted for the different variables. Often, the first 15 year period after marriage was marked by various levels of divergence, with a subsequent convergence after 25 years of marriage. However, late convergence was not observed for smoking or alcohol use.
Negative caregiver outcomes, measured as loss of mental health and well-being, were investigated among persons having a partner with a mental disorder (Paper 3), hearing loss (Paper 4), and dementia (Paper 5). For these purposes, the data were matched with information about mental health and hearing loss collected for the Nord-Trøndelag Hearing Loss Study and information about dementia diagnoses collected by the Health and Memory study. Spouses of persons with mental disorder and dementia scored significantly lower on subjective well-being and higher on symptoms of anxiety and depression than did the remaining population. Estimated group differences were ranging from 0.3-0.5 fractions of a standard deviation (SD). No negative effect of being a spouse was observed in the case of audiometric hearing loss. However, using self-reported hearing loss as a primary predictor, spouses scored significantly higher on mental distress and lower on well-being compared with spouses in the remaining population. Effect sizes were small, corresponding to 0.06-0.13 SD.
The results presented in this thesis indicate that spouses influence each other to some extent; however, many associations between couples appear to be present from the very beginning of a relationship. This thesis adds to the existing body of knowledge pertaining to illness and disability in marital dyads by estimating negative outcomes in a large population-based sample. As the population ages, one may expect an increase in the proportion of people experiencing illness or disabilities in their partner. Knowledge about initial partner similarity, couple convergence, and the consequences of caregiving burdens provides important information for the development of effective interventions aimed at improving the situations of individuals with various disabilities or illnesses, as well as affected spousal caregivers.