Adopting a complex systems approach to functional ageing: bridging the gap between gerontological theory and empirical research.

Functional ageing, related to but distinct from biological and environmental systems, is defined as the changes in physical, psychological, cognitive, and social functioning, as well as behavioural factors of individuals as they age. In this Personal View, we propose that a complex systems perspective to functional ageing can show how outcomes such as quality of life and longevity, and success in prevention and treatment, emerge from dynamic interactions among these domains, rather than from single causes. We support this view in three ways. First, we explain how three key principles of complex systems science-namely, resilience, non-linearity, and heterogeneity-apply to functional ageing. Second, we show how established gerontological theories and geriatric models align with these principles. Third, we illustrate the use of novel methodological tools available from complex systems science for studying functional ageing. Finally, we offer a glossary of key concepts and recommendations for researchers to adopt this perspective in future studies on functional ageing.