Background The ability to learn, solve problems, recollect, and suitably use stored information is key to successful health and ageing. Cognitive decline creates significant challenges for patients, their families, and clinicians delivering health care. The study of the resemblance between spouses in terms of diseases and their risk factors has been employed as a popular study design to examine the role of the environment in the development of health conditions. The primary objective of this study was to understand the effect of spousal similarity on cognitive decline among older adults living in India. The secondary objective was to examine whether the duration of cohabitation influenced the strength of concordance among spouses.

Methods We used the Harmonised Longitudinal Ageing Study in India (LASI) Wave 1 for the analysis. A total of 37,676 individuals (18,838 couples) from the harmonised LASI dataset were considered for the final analysis, excluding patients already diagnosed with Alzheimer’s disease or dementia. We employed alternating logistic regression to estimate spousal concordance of cognitive health for the Indian population. Alternating logistic regression estimates pairwise odds ratios accounting for the intra-couple association. A pairwise odds ratio was produced to estimate spousal concordance of cognitive decline, controlling for covariates attributable to assortative mating and cardiovascular risk factors. The model incorporated covariates including place of residence, caste, religion, education, drinking, smoking, BMI, physical activity, and comorbidities such as hypercholesterolemia, hypertension, diabetes, and heart problems. The analysis was performed using R v4.1.3 with the ‘alr’ package v0.1.0/r72

Results The odds of developing cognitive decline among a spouse was approximately 2.5 times (OR 2.53, 95% CI 2.31-2.77) more if the other spouse was cognitively declined for the Indian population. The association between length of marriage, in years, and cognitive decline was significant (OR=1.03, 95% CI: 1.03 -1.04). With a one-year increase in the length of marriage, there was a 3% higher risk of cognitive decline among couples in the LASI dataset.

Conclusion To our knowledge, this is the first study assessing spousal concordance on cognitive decline in an Indian population. These results show that the post-marriage shared environment has a substantial effect on cognitive decline among couples. Therefore, clinicians should screen spouses once their partners are diagnosed with cognitive decline, and preventative interventions should target couples rather than individuals. Though the causal inference could not be identified due to the cross-sectional study design, a dose-response relationship was established considering the variable on length of marriage