Ever since weed started becoming legal across the country, and more and more people started indulging in what was once taboo, scientists have been mulling over the meaning of a particular observation. That observation? Why do some people use cannabis products casually while others develop problematic use patterns?

To figure it out, a team of researchers at Washington State University turned to rats, the scientists’ most trusted test subject, for some answers. More specifically, they were testing out the effects of marijuana on stressed-out rats. You know, rats who are really feeling the crunch from their demanding day jobs and crushing debt.

Their findings, which were published in a scientific journal with maybe the most preposterously pronounceable name I’ve ever come across from one of these things: Neuropsychopharmacology. Or you can read all about it on the much more approachable science news website, Eureka Alert.

The study examined cannabis-seeking behavior in rats by first building detailed behavioral and biological profiles. The animals were assessed on everything from cognition and social behavior to sex differences and reward sensitivity.

Then came the most important part: after a long day of doing stressful rat stuff, did the rats self-administer cannabis vapor by poking their noses into a port that delivered a three-second puff of the stuff straight into their bloodstream when it was offered to them one hour a day over a span of three weeks?

Like us, some rats barely touched the stuff. Meanwhile, others kept coming back. They found that rats’ stress levels were the strongest predictor of which rats became regular users.

Rats with naturally higher levels of corticosterone, the rodent equivalent of human cortisol, aka the so-called “stress hormone,” were far more likely to seek out cannabis. These rats weren’t stressed after a rough day at the office or when they were faced with a sudden, difficult challenge. That stuff didn’t matter. What mattered was the animal’s default resting stress level.

Researchers also found that rats that struggled to adapt when the rules of the experiment were tweaked were more motivated to use cannabis. They also found that lower levels of endocannabinoids, which are the body’s naturally produced compounds that help regulate mood and maintain internal balance, were also linked to higher cannabis use.

One theory they suggest is that THC may act as a substitute when those internal systems are running low, thus nudging some people toward heavier use than others.

Put all this together, along with what we’ve previously discovered about marijuana use, and it paints a picture of cannabis being used as a coping mechanism. The rats weren’t just getting high. They were self-medicating to handle the stresses of the day better.

According to the researchers, identifying baseline stress levels early in life could one day help flag people at higher risk, before coping turns into dependence.