BackgroundExisting evidence on the neuroprotective effects of social support cannot exclude reverse causality. Consequently, the social support–cognitive ageing associations could be driven more by cognitive decline than by the lack of social support. This study aimed to investigate the dynamic reciprocity between social support and cognitive function in late life. MethodsAnalyses were based on three parallel repeat measures of social support and cognition from the Whitehall II cohort, providing 10 year follow-up of 6863 participants (71% male, mean age 55·8 years [SD 6·0] at analysis baseline). Alternative dynamic hypotheses were evaluated via four bivariate dual change score models: the full coupling model estimated mutual effects of social support and cognition on subsequent changes in each other; the social causation model assumed a unidirectional effect from social support onto changes in cognition only, whereas the opposite assumption was tested by the health selection model; lastly, the no coupling model suggested independent growth of these two sets of variables. FindingsThe health selection model fitted the current analytical data the best. A leading effect from executive function on subsequent changes in practical support was identified (β=0·18, SE 0·06). Removing this effect (social causation: Δχ2=6·03, df=1, n=6863; p=0·004) or both coupling effects (no coupling: Δχ2=9·38, df=2, n=6863; p=0·009) degraded the model fit considerably. Likewise, health selection was the preferred model between executive function and confiding support: a greater cognitive function at the preceding stage was associated with less positive changes in confiding support (β=–0·11, SE 0·05) but effects from confiding support onto cognition were not statistically different from zero (β=–0·05, SE 0·06). The preferred models were similar for memory and social support. InterpretationThe present study provides empirical evidence for the health selection mechanism, suggesting that cognition is more likely to be the leading modifying factor for changes in social support, whereas there is no detectable affect the other way around. These dynamic interplays between social support and cognitive decline should be considered in constructing our age-friendly and dementia-friendly society. FundingThe Whitehall II Study is supported by grants from the Medical Research Council (grant K013351), the British Heart Foundation (grant RG/13/2/30098), and the National Institute on Aging, US National Institutes of Health (grant AG13196). JH was partly supported by the Economic and Social Research Council (grant ES/K01336X/1) and the National Institute on Aging (grant R01AG013196). The funding bodies did not play any part in the study design; the collection, analysis, and interpretation of the data; the writing of the manuscript; or the decision to submit the abstract for publication.