How long does it really take to form a habit?

Not 21 days. In the best-known study on the question, it took people an average of 66 days of daily repetition before a new behavior felt automatic, and the range was enormous: from 18 days to 254. A habit, in the psychologist's sense, is a behavior repeated so regularly that it starts to run without much conscious thought, and that takes longer than the famous three weeks.

The 21-day figure has a surprising origin. It apparently comes from Psycho-Cybernetics, a 1960 self-help book by the plastic surgeon Maxwell Maltz, who noticed that his patients took about 21 days to get used to their new faces. Nobody ran an experiment on it, but the book stretched the number to self-transformation of every kind.

The real data came in 2009, when Phillippa Lally and her colleagues at University College London followed 96 volunteers who each picked a new daily habit, like eating a piece of fruit at lunch or running for 15 minutes before dinner. Simple habits won: a daily glass of water became automatic very quickly, while 50 sit-ups before breakfast needed far more dedication. The good news for the imperfect is that missing a single day did not reduce the chance of the habit sticking.

What builds a habit is repetition in the same situation. Each time you do something in a consistent context, the link between that context and the action grows a little stronger, until the context alone is enough to set it off. That is why, according to one study by Wendy Wood, around 43 percent of what we do each day is done out of habit.