Expressive law is said to induce compliance with stated principles without a price on non-compliance. We empirically assess this proposition, by attempting to dis-entangle the impacts of a legal change (a 5p charge on use of plastic bags), on individual choices. We do so by measuring both behaviours and attitudes across the first two months of the legal change, and by comparing the impacts across neighboring jurisdictions both with and without the change. Using mediation analysis, we find that the self-reported change in internal motivation explains only about 10% of the change in behaviour. Interestingly, we find that the scale of the sanction (charge) is both irrelevant (because jurisdictions without sanctions still exhibit changed behaviour) and important (because the size of the sanction signals the reasonableness of the law).