The 21-day habit rule came from a plastic surgeon’s book preface
Maxwell Maltz was describing how long patients took to get used to a new face. When researchers finally measured habit formation, the range ran from 18 days to 254.
What ‘just go for a walk’ leaves out
The evidence on exercise and depression is weaker than the headlines and more interesting than the sceptics allow. It points at structure and supervision, not walks.























