Scientists assessed the moment of anticipation (a.k.a.
when volunteers knew they could get a few dollars richer).
"Some people may believe that marijuana is not addictive or that it's 'better' than other drugs that can cause dependence," Heitzeg said.
"But this study provides evidence that it's affecting the brain in a way that may make it more difficult to stop using it.
"For the most part, it was widely used for medicine and spiritual purposes," during pre-modern times, said Warf, a professor of geography at the University of Kansas in Lawrence.
For example, the Vikings and medieval Germans used cannabis for relieving pain during childbirth and for toothaches, he said.
In 1937, the Marijuana Tax Act put cannabis under the regulation of the Drug Enforcement Agency, criminalizing possession of the plant throughout the country.
Researchers analyzed 108 people in their early 20s (69 men and 39 women), all of whom were taking part in a larger study of substance use.
For instance: Toking up regularly could dull your emotional response and cause addiction, according to a marijuana study from the University of Michigan Health System.
The science is still, well, hazy—but some research is starting to give us an idea of what exactly weed does to the human body.
Before each round, they were told they could win 20 cents or —or they might lose that amount, or have no reward or loss.
"It likely flourished in the nutrient-rich dump sites of prehistoric hunters and gatherers," Warf wrote in his study.