Performance reporting in sunshine regulation is subjected to disclosure biases, because agents may game the regulation or encounter difficulties in complying. These biases limit the appraisal of the impact of sunshine regulation on performance. We investigate the behavioral causes of such disclosure biases by focusing on transaction cost economizing and opportunism. We provide an original methodology to take into account information asymmetries in principal–agent relationships. We focus on water utilities management in France. Our data set includes 795 observations covering water utilities and performance indicators characteristics. It allows for comparisons of revealed and observed performances and identifies different types of disclosure biases. Findings indicate that opportunism is a significant motivation for disclosure biases, while, unexpectedly, transaction costs are not a direct trigger of disclosure biases.