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Cannabis smokers warned they risk poorer exam grades

Dutch study finds mathematics results suffer most from dope consumption – findings sure to fuel debate over steps towards legalisation

Cannabis smokers in a Dutch coffee shop. Research in Maastricht looked at the effect on course grades. Photograph: Yadid Levy/Alamy

Cannabis smokers in a Dutch coffee shop. Research in Maastricht looked at the effect on course grades. Photograph: Yadid Levy/Alamy

First published on Sat 11 Apr 2015 11.40 BST

If you want to do well in your exams, especially maths, don’t smoke dope.

This is the finding of a unique study that is likely to be fiercely debated by those in favour of and those against the liberalisation of cannabis laws. Economists Olivier Marie of Maastricht University and Ulf Zölitz of IZA Bonn examined what happened in Maastricht in 2011 when the Dutch city allowed only Dutch, German and Belgian passport-holders access to the 13 coffee shops where cannabis was sold. The temporary restrictions were introduced because of fears that nationals from other countries, chiefly France and Luxembourg, were visiting the city simply to smoke drugs, which would tarnish its genteel image.

After studying data on more than 54,000 course grades achieved by students from around the world who were enrolled at Maastricht University before and after the restrictions were introduced, the economists came to a striking conclusion. In a paper recently presented at the Royal Economic Society conference in Manchester they revealed that those who could no longer legally buy cannabis did better in their studies. The restrictions, the economists conclude, constrained consumption for some users, whose cognitive functioning improved as a result.

“The effects we find are large, consistent and statistically very significant,” Marie told the Observer. “For example, we estimate that students who were no longer able to buy cannabis legally were 5% more likely to pass courses. The grade improvement this represents is about the same as having a qualified teacher and, more relevantly, similar to decreases in grades observed from reaching legal drinking age in the US.”

For low performers, there was a larger effect on grades. They had a 7.6% better chance of passing their courses.

Interestingly, Marie and Zölitz found the effects were even more pronounced when it came to particular disciplines. “The policy effect is five times larger for courses requiring numerical/mathematical skills,” the pair write.

This, they argue, is not that surprising. “In line with how THC consumption affects cognitive functioning, we find that performance gains are larger for courses that require more numerical/mathematical skills,” Marie said. THC – tetrahydrocannabinol – is the active ingredient in skunk cannabis, which some studies have linked with psychosis.

The groundbreaking research comes at a significant moment. The clamour for liberalisation of cannabis laws is growing. In Germany, Berlin is considering opening the country’s first legal cannabis shop. Uruguay plans to be the first nation in the world to fully legalise all aspects of the cannabis trade. In the US, more than 20 states now allow medical marijuana use, while recreational consumption has become legal in Alaska, Oregon, Washington and Colorado.

But, as Marie and Zölitz observe in their paper: “With scarce empirical evidence on its societal impact, these policies are mainly being implemented without governments knowing about their potential impact.

“We think this newfound effect on productivity from a change in legal access to cannabis is not negligible and should be, at least in the short run, politically relevant for any societal drug legalisation and prohibition decision-making,” Marie said. “In the bigger picture, our findings also indicate that soft drug consumption behaviour is affected by their legal accessibility, which has not been causally demonstrated before.”

The research is likely to be seized upon by anti-legalisation campaigners. But Marie was at pains to say the research should simply be used to raise awareness of an often overlooked aspect of drug use: its impact on the individual’s cognitive ability. “If marijuana is legalised like it is in many states in the US, we should at least inform consumers about the negative consequences of their drug choices.”

It will also feed into the debate about THC levels in cannabis, which are becoming ever stronger. Levels of THC in marijuana sold in Maastricht’s coffee shops are around double those in the US.

“Considering the massive impact on cognitive performance high levels of THC have, I think it is reasonable to at least inform young users much more on consequences of consuming such products as compared with that of having a beer or pure vodka,” Marie said.

History suggests that prohibition often results in the illicit drug or alcohol trade producing ever stronger products. Campaigners for liberalisation argue that it could help bring THC levels down and allow users to know what they are buying. The authors concede that their findings could turn out to be different if they were to replicate their study in a country that did not have restrictions on cannabis use.

Marie said his work had helped inform his discussions with his teenage son. “I have a 13-year old boy and I do extensively share this with him as a precautionary measure so that he can make the best informed choice if he is faced with the decision of whether to consume cannabis or not.”

Dutch study finds mathematics results suffer most from dope consumption – findings sure to fuel debate over steps towards legalisation

Marijuana use by teenagers does not result in a lower IQ or worse exam results, study finds

The Avon Longitudinal Study of Parents and Children followed 2,612 children born in the Bristol area in 1991 and 1992

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Moderate cannabis use among teenagers may not lead to a lower IQ and poorer exam results, a large study of UK schoolchildren has found.

The study warns however that teenagers who regularly use cannabis, at least 50 times by the age of 15, may tend to do worse in exams.

The Avon Longitudinal Study of Parents and Children followed 2,612 children born in the Bristol area in 1991 and 1992.

Each child had their IQ tested at the ages of eight and again at 15, when they were also given a survey on cannabis use.

Examination results were also looked at as scientists analysed whether cannabis use had affected intellectual and educational performance.

Lead researcher Claire Mokrysz, of University College London, said the findings suggested cannabis “may not have a detrimental effect on cognition”, once other related factors, including smoking cigarettes and drinking alcohol, had been taken into account.

She said: “This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behaviour and personal history typically associated with cannabis use, rather than cannabis use itself.

“People often believe that using cannabis can be very damaging to intellectual ability in the long-term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations.

“Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices. It’s hard to know what causes what – do kids do badly at school because they are smoking weed, or do they smoke weed because they’re doing badly? This study suggests it is not as simple as saying cannabis is the problem.

Cannabis around the world

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Cannabis around the world

Morocco

Cannabis around the world

Colorado

Cannabis around the world

Oakland

Alain Jocard/AFP/Getty Images

Cannabis around the world

Seattle

Cannabis around the world

China

Cannabis around the world

Uruguay

Cannabis around the world

Colorado

Cannabis around the world

Berlin

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Cannabis around the world

Cannabis around the world

Amsterdam

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Merseyside

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San Francisco

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Spain

‘This is a potentially important public health message – the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviours. However the finding that heavier cannabis use is linked to marginally worse educational performance is important to note, warranting further investigation.’

The study was presented at the annual congress of the European College of Neuropsychopharmacology (ECNP) in Berlin.

ECNP chairman Professor Guy Goodwin said: ‘This is a potentially important study because it suggests that the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself.’

Earlier this month a study said almost as many teenagers and young adults smoke cannabis as cigarettes.

At the time, Professor Wayne Hall, from the National Addiction Centre at King’s College London, said the drug’s negative effects should not be underestimated after looking at two decades of data.

“Over the past 20 years, we have seen a large increase in the number of people smoking cannabis,” he said.

But the risk of addiction is higher with teenagers, with one in six who regularly use it becoming dependent and one in ten adults.

1 /3 Marijuana use by teenagers does not result in a lower IQ

Marijuana use by teenagers does not result in a lower IQ

Marijuana use by teenagers does not result in a lower IQ

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Marijuana use by teenagers does not result in a lower IQ

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The Avon Longitudinal Study of Parents and Children followed 2,612 children born in the Bristol area in 1991 and 1992