How long a habit takes

What was measured

Phillippa Lally and colleagues at University College London published “How are habits formed: Modelling habit formation in the real world” in the European Journal of Social Psychology (online in 2009, in print in 2010). The study had 96 volunteers. Each chose one simple action, such as eating, drinking or doing something, and carried it out every day in the same situation, for example “after breakfast”. For twelve weeks, participants recorded whether they had done the action and answered questions about how automatic it felt.

The authors were not looking for “the day the habit is ready” but for a curve: how automaticity grows with repetition. The curve was calculated for each participant separately. There was enough data for analysis for 82 people, and the model could be fitted for 62.

What came out

Automaticity rose faster at first and more slowly later: the curve gradually levels off at a plateau. The time to reach 95 % of that plateau ranged from 18 to 254 days depending on the person, with a median of 66 days. Observation lasted 84 days, so longer values are an extension of the model curve, not a direct measurement. The authors’ main point is the range itself: for some people it takes weeks, for others months.

One more detail from the paper: missing a single occasion did not materially affect the process. This is the finding of one study, not a rule for everyone, but it suggests that repetition overall matters, not an unbroken chain of days.

Where “21 days” comes from

The number 21 is usually traced to the book “Psycho-Cybernetics” (1960) by the surgeon Maxwell Maltz. Maltz described patients after cosmetic surgery who needed time to get used to their changed appearance, and wrote that an old self-image needs “a minimum of about 21 days” to give way to a new one. He was writing about getting used to a new face, not about forming habits. In retelling, the word “minimum” was lost, and an observation turned into a deadline.

What follows from this

If an action still takes effort and a reminder after two or three weeks, that is to be expected. In the study by Lally and colleagues, automaticity for most participants was only building up at that point.

A missed day is no reason to start over. It is more useful to return to the action in the same situation the next time.

Participants tied the action to a fixed cue: the same action after the same event. Habits in the app are set up the same way: “after what” and “what”.

How this works in Metamorphosis

For the first three weeks, the habit screen shows one unobtrusive line noting that habits take different amounts of time to settle in, with a link to this article. A single missed day does not break the streak: it allows one day a week without a check mark. Yesterday can still be checked off later.