Researchers at the University of Colorado are embarking on one of the largest studies on how marijuana affects mental health. And they're using twins to do so.

The National Institute on Drug Abuse awarded the University of Colorado a $5.5 million grant to conduct research on how marijuana use affects mental health, and whether legalizing cannabis led to changes in use of the drug and its effects. The researchers are conducting the study on 5,000 twins aged 23 to 29, 2,500 in Colorado (where marijuana was legalized in 2014) and 2,500 in Minnesota (where marijuana is still illegal).

The scientists have already been following the twins for 15 to 20 years and monitoring their use of marijuana, alcohol and other drugs, their social behavior and mental health. They are now going to collect data from the 2,500 twins living in Colorado to see if their behavior has changed since the state legalized marijuana in 2014. The scientists are including the twins in Minnesota to see if non-legalization reasons can be determined for any changes in behavior they find in the ones living in Colorado.

If you're wondering why the scientists are using twins, it's because they also want to determine if genetic or environmental factors can make people more susceptible to the changes brought on by marijuana legalization.

The scientists are hoping their study will help paint a clearer picture about how marijuana legalization affects the people who live in those areas.

“Some people will be fine. Some people may benefit. But for a subset of people, we suspect there will be adverse consequences,” says one of the lead researchers.

(h/t University of Colorado)