Evidence of Bias in Cannabis-Harm Studies

By David Jenison on March 6, 2018

Researchers who study the potential harms of cannabis are often biased. So says a new study in Archives of Clinical Neuropsychology that confirms something cannabis consumers have known since the first plants emerged from the ground following the last Ice Age. 

The study, titled "Don't Judge a Book by its Cover," involved 41 cannabis consumers and 20 non-users assessed using various neuropsychological measures. Before initiating the tests, however, the Palo Alto University researchers had to guess which participants were stoners. 

That must have been fun. Did the Kyuss shirts give them away?  

The researchers guessed that 37 of the 61 participants were chronic cannabis consumers, and 75 percent of their guesses were correct. If graded like the university students they teach, the researchers would have received a C on their report card. Of course, the guesswork comes with relatively predictable stereotypes, e.g., a slightly chubby pasty-white dude that looks like a guest commentator on Fox News probably doesn't "smoke the pot." Still, just as a researcher shouldn't assume said white dude also has anger issues, there should be no assumption that cannabis smokers have neuropsychological issues.  

As you might expect, this was not the case with the Archives study. 

"Examiner judgments of cannabis user status predicted performance on two measures, as individuals judged as cannabis users obtained lower scores than those judged as non-users," the research found. "Judgments of cannabis user status predicted performance even after controlling for actual user status, indicating vulnerability to examiner expectancy effects."

In layperson speak, stoners and non-stoners alike fared worse on neuropsychological assessments if the medical professionals conducting the tests thought the person smoked cannabis. Conversely, stoners and non-stoners both fared better on the assessments if the test givers thought the person did not smoke cannabis. So at least according to this study, research into potential cannabis harm commonly involves an expectancy bias that skews the results toward negative outcomes. 

"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," the researchers concluded. "These results demonstrate the need for expectancy effect research in the neuropsychological assessment of all populations, not just cannabis users."

In other words, it's time for clinical research into cannabis harm to exit the Ice Age. 

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