Phillippa Lally
Phillippa Lally is a British health psychologist working on habitual behaviour and how it can be used in behaviour-change interventions. She is the first author of Lally, van Jaarsveld, Potts and Wardle (2010), the study that supplies almost everything anybody knows about how long habits take to form in the real world: a median of 66 days to automaticity, a range of 18 to 254 days, and, in the same paper, the finding that a single missed day did not measurably damage the habit curve. The median travels everywhere. She has spent years correcting how it is quoted.
Who they are
She carried out the habit-formation work at University College London, in the Research Department of Behavioural Science and Health, where she was later a Senior Research Fellow. In January 2023 she moved to the University of Surrey as a Senior Lecturer, where she co-directs the Habit Application and Theory Research Group with Benjamin Gardner. Much of her current work concerns energy-balance behaviours in people living with and beyond cancer, and she co-leads the European Health Psychology Society's Habit Special Interest Group.
The 2010 paper is the reason the vault holds a general habit formation note at all. It is the field's one real-world dataset on the question everybody asks.
Key papers
Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). "How are habits formed: modelling habit formation in the real world", European Journal of Social Psychology, 40(6), 998 to 1009. First author.
Design: 96 volunteers, each choosing one eating, drinking or activity behaviour to perform daily in a consistent context, tracked for twelve weeks with the Self-Report Habit Index.
Findings: automaticity rose asymptotically, with a median of 66 days to plateau and a range of 18 to 254 days across individuals and behaviours. More complex behaviours took longer. A single missed day did not measurably damage the habit curve.
Limits worth keeping with it: the sample is small, self-selected and self-reported, the behaviours were mostly simple, and the study measures how automaticity grows rather than how durable a habit is afterwards.
Key ideas
"66 days to form a habit" is a misquotation twice over. It is a median rather than a figure that applies to a given person, and the spread around it is more than eight-fold. Anyone quoting the number without the range is quoting half a result.
The shape matters more than the number. Automaticity rises steeply and then flattens, so the early repetitions buy more than the later ones and the hardest stretch is the one before anything feels easier.
The missed-day finding is the part nobody repeats and the most useful thing in the paper. A single omission did not set the curve back, which is the strongest available evidence against the reasoning that ends most attempts at a routine. See all-or-nothing thinking.
The evidence for starting small belongs to this literature. Where an argument needs evidence rather than a framework, the citation is Lally and not a trade book.
Concepts on this site
Habit Formation, the general concept her study anchors
The Habit Loop, the popular model her data is usually cited alongside
All-or-Nothing Thinking, contradicted by her missed-day finding
Exercise Adherence, the applied domain
Sources
- Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). European Journal of Social Psychology, 40(6), 998 to 1009.
- Vault verification record, 2026-09-11. Citation confirmed, and the range and missed-day findings recorded as the two parts that must travel with the median.