When initially confronted with the Monty Hall dilemma (MHD), people show a very strong tendency to stick with their initial choice, although switching maximizes winning chances. Previous research demonstrated that certain interventions helped participants to discover and apply the optimal strategy, but generally failed to increase participants’ understanding of the MHD solution. An exception on the latter finding is DiBattista’s (2011) digital learning environment study, reporting that the majority of participants who used the learning environment learned to understand the MHD solution. However, a major shortcoming was DiBattista’s (2011) methodology, which did not allow to infer causal relations and to conclude which (combination of) manipulation(s) was most important for participants’ understanding of the MHD solution. The aim of the present study was to fill this research gap by conducting a controlled randomized experiment with an analogous digital learning environment. Participants were high-school students between 16 and 19 years old. The results showed that receiving explanation about the MHD solution was the most important manipulation to improve understanding. Implications for education in (posterior) probability are discussed