Memory is tricky, especially when drugs come into play. These two seemingly obvious facts could have big implications for the criminal justice system. For decades, psychologists have known that memories can be manipulated, making eye-witness testimony notoriously unreliable in courtrooms. But with the rising tide of marijuana liberalization in the United States and around the world, the time for the criminal justice system to reckon with the effects of marijuana on eye-witness testimony is long overdue. Fortunately, a team of psychologists is on it.

In a paper published on April 19 in the journal Applied Cognitive Psychology, a team of researchers at Vrije Universiteit Amsterdam and Florida International University found that people who witnessed a crime under the influence of cannabis and were interviewed about it while under the influence were much less able to recall accurate details about the incident than their non-stoned counterparts. But when it came to identifying suspects in a photographic lineup, there was no significant difference between the two groups. And in fact, the researchers found that the pot-smoking participants were more confident in their assessments, showing a positive correlation between confidence and accuracy.

Taken altogether, these results show that the effects of cannabis on eye-witness testimony are not clear-cut.

smoking marijuana
Researchers found that people under the influence of marijuana recalled fewer accurate details about a crime than their sober counterparts.

To reach these results, the researchers conducted a study of 63 stoned volunteers and 55 non-stoned volunteers recruited either leaving or entering coffee shops in Amsterdam, businesses that are well known for selling marijuana products. They had volunteers watch a video of a convenience store robbery and then recall details about it after completing a filler task consisting of a 2-minute Sudoku puzzle. During the recall phase, they were first asked to simply recall anything they could, and then they were asked a set of open-ended questions.

In this part of the study, the people who reported having used cannabis that day were significantly less capable of remembering accurate details of the robbery than the people who had not used that day.

“Our finding that participants under the influence of cannabis recalled fewer correct details is consistent with the general literature on the effects of cannabis, which has shown that cannabis reduces immediate recall,” write the researchers, led by Annelies Vredeveldt, Ph.D., an assistant professor at the Department of Criminal Law and Criminology at VU University Amsterdam.

photographic lineup
Participants were shown a photographic lineup like this one and asked to identify the suspect from the robbery video.

But this is where the picture got a bit more complicated. When participants were shown a photographic lineup and asked to identify the suspect from the video, there was no significant difference between the stoned participants and the non-stoned participants. The researchers write that this finding was consistent with a previous study with a smaller sample size, as well as with the finding that cannabis impairs recall memory much more than it impairs recognition memory.

Additionally, the smokers who were confident in their identification of the suspect were more likely to be accurate. The study’s authors propose an explanation:

This finding is not in line with the hypothesis that intoxicated participants would compensate for anticipated poorer performance by adjusting their confidence ratings downward. It is in line, however, with the hypothesis that cannabis leads witnesses to become more focused on their cognitive processes, thus leading accurate witnesses to weight their recognition experiences particularly heavily and increase their confidence.

hemp, cannabis
The effects of cannabis on memory are not straightforward.

Long story short, the researchers found that there’s no straightforward effect of cannabis intoxication on eye-witness testimony. While it appears to hurt recall memory, it also seems to leave recognition memory of people unhindered, which the researchers note is arguably “the most forensically relevant information in a place investigation, as it may lead the police to a suspect or contribute to the evidence incriminating or exonerating a suspect.”

One complicating factor of the research is that the intoxicated subjects were stoned while they witnessed the crime and while they were being interviewed about it. The researchers point out that future studies should have people who witness a crime interviewed sober, and vice versa, to tease apart whether cannabis seems to have a greater effect on encoding or retrieval of memories. But for now, the researchers have begun filling in a gap in the research, one that could have significant effects on the credibility of eye-witness testimony, one of the most controversial elements of the legal system.