Research paper
Lally P, van Jaarsveld CHM, Potts HWW, Wardle J. How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology. 2010;40(6):998-1009. doi:10.1002/ejsp.674
Why we cite it
The empirical study that reported the 66-day median — and, more importantly, showed why that number is a median, not a rule. Lally et al. tracked 96 volunteers (82 with sufficient data) who each chose an eating, drinking, or activity behavior to perform daily in the same context for 12 weeks, measuring automaticity via the Self-Report Habit Index. The asymptotic curve they fitted revealed that automaticity plateaus around 66 days at the median — but ranges from 18 to 254 days. The median is over the 39 participants whose curves fitted well, and the paper does not show that behaviour type explains the range. Three results follow: habit formation has a timeline measured in months; the curve is front-loaded; and one missed day did not ”materially” affect the process. *The exercise group’s ”one and a half times longer” is not a finding to set expectations by*: it was not significant (p = 0.328), and the authors say the study was not powered for sub-group analyses.
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