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Don’t Judge a Book by its Cover: Examiner Expectancy Effects Predict Neuropsychological Performance for Individuals Judged as Chronic Cannabis Users

Objective
The experimenter expectancy effect confound remains largely unexplored in neuropsychological research and has never been investigated among cannabis users. This study investigated whether examiner expectancies of cannabis user status affected examinees' neuropsychological performance.


Method
Participants included 41 cannabis users and 20 non-users. Before testing, examiners who were blind to participant user status privately rated whether they believed the examinee was a cannabis user or non-user. Examiners then administered a battery of neuropsychological and performance validity measures. Multiple regression analyses compared performance between examinees judged as cannabis users (n = 37) and those judged as non-users (n = 24).


Results
Examiners' judgments of cannabis users were 75% accurate; judgments of non-users were at chance. After controlling for age, gender, and actual user status, examiner judgments of cannabis user status predicted performance on two measures (California Verbal Learning Test-II, and Trail Making Test B; p < .05), as individuals judged as cannabis users obtained lower scores than those judged as non-users.


Conclusions
Examiners' judgments of cannabis user status predicted performance even after controlling for actual user status, indicating vulnerability to examiner expectancy effects. These findings have important implications for both research and clinical settings, as scores may partially reflect examiners' expectations regarding cannabis effects rather than participants' cognitive abilities. These results demonstrate the need for expectancy effect research in the neuropsychological assessment of all populations, not just cannabis users.