What the study actually found
Phillippa Lally and colleagues at University College London asked 96 volunteers to choose one new daily behaviour, such as eating a piece of fruit with lunch, drinking water after breakfast or going for a run before dinner, and to do it in the same situation every day for 12 weeks. Each day they rated how automatic it felt.
The ratings rose quickly at first and then levelled off, like the curve above. Of the 82 people who gave enough data, the curve could be fitted for 62 and fitted well for 39. For those 39, the time to reach 95% of their own plateau had a median of 66 days and ranged from 18 to 254 days. The paper, "How are habits formed: Modelling habit formation in the real world", was published in the European Journal of Social Psychology in 2010.
Where "21 days" came from
Plastic surgeon Maxwell Maltz wrote in Psycho-Cybernetics (1960) that it took "a minimum of about 21 days" for people to get used to a new face or a lost limb. Over decades of retelling, "a minimum of about" was dropped. Three weeks is a fine first milestone, but don't be discouraged if a habit still takes effort on day 22. Watch how flat the pink line still is at the dashed line when you choose the slower habits.
The 21-day rule survived because almost nobody went back to Maltz's actual sentence. Other things most of us only half-know, such as why the Moon waxes and wanes, get the back-to-the-mechanism treatment in ahaboo's narrated explainers.
Three things that speed it up
- Same cue, same place. Habits are links between a situation and an action. Stack the new habit onto something you already do (habit stacking).
- Start simpler. The slower curves above belong to harder behaviours. Shrink it until it's easy, then grow it.
- Don't panic about one miss. Tick the box on the habit tracker tomorrow; the curve barely notices.