We measure the behavioural and motivational impacts of a legislative change in England that required supermarkets to charge for new plastic carrier bags they issued. Using a difference-in-difference estimator, we find that the treatment group used 1.7 less new plastic bags per shopping trip after seven weeks. We also find evidence of motivational 'crowding in'. That is, we find increased motivation to reduce plastic bag use and acceptance of the government's role in regulating their use. Using mediation analysis we find that the price effect of the charge grows over time, whereas the internal motivation effect falls (in relative terms). Seven weeks after the legislation came into force the price change explains 90% of the reduction in new plastic bags used, while the change in motivation explains only 10% of the reduction.