How habits form
In a 2010 study, a new daily habit took a median of 66 days to reach 95 percent of its final level of automaticity. The range was 18 to 254 days.
You probably do not decide to brush your teeth each morning. You walk into the bathroom and it happens. That is a habit: a behaviour triggered by a context with little conscious thought. OpenStax describes habits in drive theory terms: once a behaviour has reduced a drive, we are more likely to use it again when that drive returns. Modern habit research puts the weight on context. Repeat the same action in the same situation and the situation alone begins to start it.
The 66-day study
The curves rose quickly at first and then levelled off, much like the forgetting curve in reverse. The researchers measured how long it took each person to reach 95 percent of their own plateau. The median was 66 days, but individuals ranged from 18 to 254 days. Simple behaviours like drinking water tended to become automatic faster than exercise. And one finding is especially practical: missing one opportunity did not materially affect the habit formation process. A single missed day does not reset the count.
The "21 days" myth and the limits of the study
You may have heard that a habit takes 21 days. The figure is usually traced to a 1960 book by Maxwell Maltz, a plastic surgeon, who wrote that patients took about 21 days to get used to a new face; that was never a habit study. Lally's team found only one statement in the research literature on how long habit formation takes (Ronis, Yates and Kirscht, 1988: a behaviour done at least 10 times and at least twice a month), and their own data show that it takes much longer and that a single number is misleading. The study has limits too. Automaticity was self-reported, the group was small, and only 39 people produced clean curves. Treat 66 days as a rough guide that says "longer than most people expect, and it varies".
Building a study habit
Put the research together and a plan follows. Pick one small action, such as "review my due flashcards", and tie it to a fixed context, such as right after breakfast. Keep it small enough to do on a bad day. Expect weeks, not days, before it feels automatic, and do not treat one missed morning as failure. The flashcards on this site help with the context part: they only show cards that are due, so the daily action stays short, often a few minutes.
| Habit | Behaviour triggered by a context with little conscious thought |
| Lally et al. (2010) median | 66 days to 95% of plateau |
| Lally et al. (2010) range | 18 to 254 days |
| Missing one day | Did not materially affect habit formation |
| Context cue | A fixed time or place that starts the habit, e.g. after breakfast |
- Habits are context-triggered behaviours built by repetition.
- Lally et al. (2010): median 66 days, range 18 to 254.
- One missed opportunity did not materially matter.
- There is no reliable "21 days" rule.
- What was the median number of days to reach 95% of plateau automaticity in Lally et al. (2010)?
- How many days separate the fastest (18) and slowest (254) individuals?
- Of 96 volunteers, 82 gave enough data. How many did not?
- How long was the Lally study?
- 21 days
- 6 weeks
- 12 weeks
- 1 year
- What did missing one opportunity do?
- Reset progress to zero
- Did not materially affect habit formation
- Doubled the time needed
- Ended the habit
- Which kind of behaviour tended to become automatic faster?
- Simple ones like drinking water
- Exercise
- They were all identical
- What is a good context cue for a study habit?
- "Whenever I feel like it"
- "Right after breakfast"
- "Someday"
- "When the exam is close"
- Research shows habits reliably form in 21 days.
- Yes
- No
- Which is a limitation of the Lally study?
- Automaticity was self-reported
- It used animals
- It lasted ten years
- It had thousands of people
Sources for this course (15)
- OpenStax Psychology 2e (openstax.org/books/psychology-2e)
- sections 6.2 Classical Conditioning (Pavlov 1927; Garcia and Koelling 1966; Rescorla and Wagner 1972; Watson and Rayner 1920)
- 6.3 Operant Conditioning (Thorndike 1911 law of effect; Skinner 1938; reinforcement and punishment; shaping; primary and secondary reinforcers; schedules; Skinner 1953 on gambling; Laskowski et al. 2019; Tolman latent learning)
- 6.4 Observational Learning (Bandura, Ross and Ross 1961; attention, retention, reproduction, motivation; Yamamoto, Humle and Tanaka 2013; Anderson et al. and Ferguson on media violence)
- 10.1 Motivation (Deci, Koestner and Ryan 1999; Yerkes and Dodson 1908; Bandura 1994 self-efficacy; Maslow 1943 and its criticism; drive theory and habits)
- 8.4 Ways to Enhance Memory. Lally, van Jaarsveld, Potts and Wardle (2010) European Journal of Social Psychology 40, 998-1009, doi 10.1002/ejsp.674 (96 volunteers, 12 weeks, 82 with enough data, 39 good model fits, median 66 days to 95% of plateau, range 18 to 254 days, missing one opportunity did not materially affect habit formation)
- Hagger et al. (2016) Perspectives on Psychological Science 11, 546-573, PubMed 27474142 (23 labs, N = 2,141, d = 0.04, 95% CI -0.07 to 0.15)
- Dang et al. multilab replication, PMC8186735 (12 labs, N = 1,775, d = 0.10)
- Dunlosky et al. (2013) Psychological Science in the Public Interest 14(1)
- PubMed 26173288 (10 techniques; high utility: practice testing, distributed practice; moderate: elaborative interrogation, self-explanation, interleaved practice; low: summarization, highlighting, keyword mnemonic, imagery for text, rereading)
- Pashler, McDaniel, Rohrer and Bjork (2008) PSPI 9(3) 105-119, PubMed 26162104. Mueller and Oppenheimer (2014) Psychological Science, PubMed 24760141
- Morehead, Dunlosky and Rawson (2019) Educational Psychology Review replication
- Urry et al. (2021) Psychological Science direct replication of Study 1. Roediger and Karpicke (2006) Psychological Science 17(3)
- Card intervals come from this site's own flashcard code (0, 1, 3, 7, 14, 30 days)
- Everyday scenes are made up for teaching. General information only.
Checked: 2026-10-17