The claimEvidence B · meta-analysis
It takes 21 days to build a habit
The evidence
From a 1960 plastic-surgery observation, not a habit study; the real median is ~66 days, range 18-254 (Lally 2010).
The mechanism, in brief
Twenty-one days to build a habit is a widespread claim. It comes from a 1960 observation by a plastic surgeon (Maltz): his patients took about 21 days to get used to their new appearance. That was not a study of habits.
Sources (2)
- Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). How are habits formed: modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998-1009. Tracking daily behaviors over 84 days, time to reach automaticity had a median of about 66 days (range about 18-254); missing a single opportunity did not meaningfully impair formation.
- Linardon, J., & Mitchell, S. (2017). Rigid dietary control, flexible dietary control, and intuitive eating: differential relationships with disordered eating and body image. Eating Behaviors, 26, 16-22. Cross-sectional, 372 community adults. Rigid dietary control was associated with more disordered eating; intuitive eating uniquely and consistently predicted less disordered eating and fewer body image concerns; flexible control was mixed (higher body image concerns, and lower disordered eating only once rigid control was accounted for).