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Background: Few studies examining the cognitive effects of chronic cannabis use utilize research designs where examiners are blind to user status. Even in those that do, researchers may be able to guess the user status of participants, leaving these studies vulnerable to the expectancy effect confound.Objectives: The present study examined the ability of neuropsychologists (those who would conduct research on the cognitive effects of cannabis) to differentiate cannabis users and nonusers based on physical appearance from photographs.Method: We recruited 84 participants from an international neuropsychology listserv. The sample was 59.5% female and 95.2% Caucasian, with a mean age of 41.39 years (range 26–65). Each neuropsychologist rated 25 target faces (12 cannabis users, 13 nonusers) on a Marijuana Use Likelihood Index based upon individuals' photographs.Results: Results indicate a main effect of user group, as neuropsychologists ascribed higher ratings to cannabis users on the Marijuana Use Likelihood Index, suggesting they perceive them as more likely to be users, relative to nonusers. Results also demonstrated a main effect of gender, as males received higher user ratings than females, and a significant main effect of rater gender, as female raters were more likely to rate individuals as cannabis users relative to male raters.Conclusions/Importance: The results demonstrate the importance of assessing researchers' expectations when studying the effects of chronic cannabis use, as even those designs that keep examiners blind to participant user status may be vulnerable to expectancy effects if examiners are able to guess user status based upon appearance alone. [ABSTRACT FROM PUBLISHER]