Does weed make you dumb?

The idea that regular cannabis use can leave you with lower intelligence is fairly common—and can even be supported by some scientific research. But a new twin study is coming to cannabis’ defense, debunking this old idea with stronger evidence.

This notion that cannabis causes lower cognitive functioning isn’t completely unsupported. While the idea has been floating around in anti-cannabis propaganda for decades, its scientific basis can largely be traced back to a 2012 longitudinal study that found that cannabis use in adolescence was tied to lower IQ and cognitive functioning scores in young adulthood. The study followed participants from age 13 to 38, interviewing and testing them to assess IQ, executive functioning, and cannabis use patterns. Those who started using cannabis during adolescence saw drops in their cognitive scores later on. And of these subjects, those who used the most cannabis use saw worse drops in IQ.

The evidence might seem strong that using cannabis leads to cognitive decline, but a recent 2019 twin study is questioning the methodology that led to the 2012 results, suggesting that we’ve been making unfounded assumptions about cannabis’ role in IQ drop.

Their study suggests that cannabis use doesn’t cause cognitive decline at all. Instead, they found that genetic and environmental factors can lead to both the lowered IQ scores and an increased likelihood to use cannabis.

So how did these two studies produce such different results? It primarily has to do with the different methodologies used in studying this question.

While the 2012 study clearly shows a correlation between cannabis use as a teen and lower IQ scores as a young adult, it can’t show whether cannabis actually caused changes in IQ. Instead, these results leave open the possibility that something other than cannabis is causing both these IQ changes and increasing the likelihood to use cannabis. Factors like lower socioeconomic status, tobacco use, and even simply being in pain are good suspects, as they are correlated with both cannabis use and lowered cognitive scores.

Some researchers point out that cannabis use can increase the likelihood of a teen associating with “deviant peers” who discourage academic work and encourage other substance use. But there are countless genetic and environmental factors that could be confusing the data.

Read the full article at Leafly