For how we live.
Observational studies tie fitness to slower cognitive decline. When researchers randomise older adults to exercise, the cognitive payoff shrinks — in a Cochrane review of healthy people, to nothing.
A diet built to protect the brain looked powerful when researchers watched who chose to eat it. When they assigned it in a three-year trial, both groups improved by almost exactly the same amount.
A meta-analysis of 44 cohorts covering 1,864,274 people found the risk signal attached to naps of 30 minutes or more. Below that threshold, no significant risk appeared.
A registered report fit identical models to 11 longitudinal studies covering 175,070 people. The subjective feeling carried the signal; the objective count of social contacts largely did not.