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 gave the world "66 days to form a habit" — 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 depending on the person and behavior complexity. For the project, this paper provides three essential messages: (1) habit formation has a real timeline measured in months, not the mythical "21 days"; (2) the curve is front-loaded, so early consistency matters most; and (3) missing an occasional day doesn't reset the clock. The study's most underappreciated finding may be the complexity effect — exercise behaviors took about 1.5× longer than eating/drinking changes, which directly informs how we set expectations for different types of behavior change.