7 think. 12 BRIGHT SECRETS: FROM EARLY NEUROSCIENCE TO CUTTING EDGE BEHAVIOURAL ECONOMICS HABIT FORMATION Behaviour becomes habitual through repetition. EDWARD THORNDIKE (1874–1949) Psychologist 1905 (Thorndike): The Law of Effect. Behaviours followed by pleasant consequences are more likely to be repeated than behaviours followed by unpleasant consequences. 2010 (Lally et al.): Habit formation modelled in the real world with eating, drinking and activity behaviours revealed that automaticity takes anything from 18 to 254 days to achieve.