Habit formation

principle · origin: study · evidence: supported

In short

In psychology, a habit is a behaviour triggered automatically by a context: a place, a time, the action that comes before it. You no longer have to decide each time. Habits form gradually, by repeating the same behaviour in the same context. The popular idea that it takes 21 days comes from Maxwell Maltz’s book Psycho-Cybernetics, not from studies. The data show that it usually takes longer and that the differences from person to person are very large.

What it says

How it works. Wood and Neal (2007) describe habits as associations learned gradually between a behaviour and the features of the context in which it usually happens, such as the place or the preceding action. Once formed, the habit is triggered by the context, without needing the intention of the moment. The associations form slowly and don’t change easily, neither when your goals change nor when you occasionally act differently.

The core study. Lally and colleagues (2010) followed 96 volunteers in everyday life. Each one chose an eating, drinking or activity behaviour to do daily in the same context, for example “after breakfast”. For 12 weeks, participants recorded every day whether they had done it and how automatic it felt. The data from 82 of them could be analysed.

Automaticity rose quickly at first, then more and more slowly, until it levelled off. The time to reach 95% of the maximum level ranged from 18 to 254 days. The median was 66 days, as the meta-analysis by Singh et al. (2024) summarizes. A practical result: missing a single day did not materially affect the process.

What newer studies say. Singh and colleagues (2024) gathered 20 studies, with 2,601 participants, on health habits: exercise, water, vitamins, flossing, diet. Four studies reported how long the habit took to form: medians of 59–66 days and means of 106–154 days, with individual variation between 4 and 335 days. The authors conclude that the data clearly contradict the 21-day idea and that most health behaviours need more like 2–5 months.

They also found that habits were stronger when people chose the behaviour themselves and when they did it in the morning. Frequency, how enjoyable the behaviour was and concrete plans also mattered.

Exercise. Kaushal and Rhodes (2015) followed 111 new gym members for 12 weeks. The minimum for forming an exercise habit was at least four workouts a week for six weeks. Consistency, simple exercises, the environment and how enjoyable they were all mattered.

Example

An example built for this text: you want to drink a glass of water every morning. The vague version is “I’ll drink more water”. The version that helps a habit form: “after I switch on the coffee maker, I drink a glass of water”, every day, in the same place. For the first weeks you’ll remember with effort. After a few months, the gesture should come by itself when you switch on the coffee maker.

How to apply it

The steps below are a practical approach we propose, based on the studies cited.

1. Tie the behaviour to a fixed context. Choose a stable trigger: a time, a place, an action you do anyway. The context is what will trigger the habit (Wood and Neal, 2007).
2. Repeat often. In Lally et al., participants who performed the behaviour more consistently had curves that better described a habit forming. In Kaushal and Rhodes, the threshold for exercise was four times a week.
3. Expect months, not weeks. The median is about two months, and for some people it takes much longer. If it still takes effort after three weeks, that’s normal.
4. Don’t give up after a missed day. In Lally et al., a single missed opportunity did not materially affect the process.
5. Choose something simple and enjoyable. Simplicity and enjoyment predicted habit formation in Kaushal and Rhodes. Singh et al. found stronger habits for behaviours the participants chose themselves.

Limits and nuances

• Few studies measure the time. Of the 20 studies analysed by Singh et al., only four reported how long the habit took to form. Most had a high risk of bias, and the authors note the small number of studies as a limitation.
• Automaticity is self-reported. All these studies measure how automatic participants feel the behaviour is, through questionnaires, not through direct observation.
• Simple behaviours. The studies used short, daily actions, such as a glass of water or a walk. For complex behaviours there is even less data.
• The curve didn’t fit everyone. In Lally et al., the model fitted 62 of 82 participants and fitted well for only 39. The 18–254 day figures describe only some of the participants.
• We read only the abstract of the Lally study. The details about it come from the abstract and from the summary in the meta-analysis by Singh et al. (2024).

Sources

• Phillippa Lally, Cornelia H. M. van Jaarsveld, Henry W. W. Potts, Jane Wardle (2010). How are habits formed: Modelling habit formation in the real world
• Wendy Wood, David T. Neal (2007). A new look at habits and the habit-goal interface
• Navin Kaushal, Ryan E. Rhodes (2015). Exercise habit formation in new gym members: A longitudinal study
• Ben Singh, Andrew Murphy, Carol Maher, Ashleigh E. Smith (2024). Time to form a habit: A systematic review and meta-analysis of health behaviour habit formation and its determinants
• Maxwell Maltz (1960). Psycho-Cybernetics

See also: Ego depletion, Implementation intentions