Findings from prior research in various content domains have indicated that Peer Assisted Study Sessions (PASS) attendees earned higher final grades than non-attendees. However, what makes PASS effective remains unknown; for example, PASS could improve short-term retention but hinder long-term maintenance of course content given that some methods that facilitate short-term retention of information hurt long-term retention. Of additional interest was what predicts students' attendance in PASS. We tracked 75 introductory psychology students' short- and long-term retention of course content via unit quizzes and a cumulative final exam, respectively, to determine if PASS attendance improved both types of retention. Results indicated that PASS attendees had significantly higher academic self-efficacy and final grades than non-attendees. Attendees outperformed non-attendees on three of the six quizzes and on the cumulative final exam, supporting both the short- and long-term effectiveness of PASS. (Contains 1 figure, 3 tables and 5 footnotes.)