The internalization of law is said to be a process that involves a change in people’s intrinsic motivation to act in accord with law’s obligations – so that it is possible to observe imposed obligations become individual choices. We empirically test for this phenomenon, by attempting to disentangle the impacts of a legal change (a 5 pence charge on use of plastic bags) on intrinsic motivation and individual choice. We do so by measuring both behaviors and attitudes before and after the legal change, and by comparing the impacts across neighboring jurisdictions without the change. Using a differences-in-differences (DID) estimator we find evidence for the internalization of law: that is, we find a significant increase in intrinsic motivation for those consumers subject to the implementation of the legislative change, and link this change in intrinsic motivation to an actual change in behavior. However, using mediation analysis we find that internalization of the law only explains around 5 to 8% of the change in behavior – the rest being attributable to the direct effect of the charge.