Abstract The legal system must provide certainty for the addressees of criminal law that their acts will not be assessed more severely than they could have been at the time when the act was committed (the lex mitior principle). At first sight, it is surprising that this guarantee is limited to statutory amendments and does not extend to the interpretation of the law. Yet, a sudden and unfavourable change in interpretation, previously widely and consistently accepted by the courts, is, from the perspective of the addressee of criminal law, not materially different from a change in the law itself. In the authors’ view, an established interpretation gives rise to legitimate expectations regarding the meaning of criminal law. Such expectations, however, are not protected in abstracto through the lex mitior principle, but only in concreto. In practice, a court may hold in an individual case that the potential offender acted under an institution of mistake of law, having been convinced that a different interpretation would apply. This study seeks to identify the rationale behind this difference in the treatment of changes in the law and changes in the interpretation. The existing approach to the protection of legitimate expectations is contrasted with alternative models, allowing for a cost–benefit analysis of their possible adoption. The analysis is grounded in Polish criminal law, which provides a useful illustration of trends widely present, at least within European criminal law.
Bielska-Brodziak et al. (Thu,) studied this question.