I found an older review article discussing the psychology behind PBL. First, more empirical research is needed in order to understand if PBL is superior to other forms of learning. But, the evidence seems to suggest that PBL results in some long lasting learning. In the short-term, PBL may actually inhibit learning. This seems to be due to the effectiveness of short-term recall related to memorization, while PBL tends to be more difficult to assess gains. In the end, however, PBL helps with retention and generalization to other contexts, as it is meant to be a way to critically think about a topic or issue.