Age is the main risk factor for cognitive decline. So with more people around the world living longer, cases of dementia are set to rise. There's currently no cure - but research suggests that happiness can reduce the impact of this awful illness. 
Psychologist Emily Willroth helped write a chapter on this topic for the 2024 World Happiness Report - and explains how making friends, helping others and engaging in fun physical activities can slow cognitive decline, even when the disease has taken hold.  See omnystudio.com/listener for privacy information.