Someone is walking behind you. Their pace is even, but you know they’re close. You quicken your step, turn the corner—are they watching? Maybe you shouldn’t leave the neighborhood any more, or better yet, the house–but even in the house, you might not be safe. You check your phone; it seems like someone is tracking your calls. They can read your texts, maybe even your thoughts.

Paranoid thoughts like these are presented in movies and culture as a symptom of severe, rare mental illnesses like schizophrenia. But some say that these kind of thoughts are far more common than you might think. According to Oxford University professor Daniel Freeman and Jason Freeman in their book Paranoia: the 21st Century Fear, in a study of 1,005 mentally healthy New York adults, 10.6 percent thought someone was “following or spying on them.” In a sample of 1,202 British college students, the number was even higher: 29 percent. Similar percentages of mentally healthy adults in France and the U.K. believed that there were social conspiracies against them, with a reason to mistrust the intentions of those around them.

These mild delusions are less disruptive than clinical paranoia, and have been providing great fodder for researchers hoping to gain insight into what what happens in the mind of someone with mental illness. One technique for studying paranoid thinking is familiar for anyone who has had awkward times with a “mellow” pot brownie that was anything but: Researchers have been inducing these thoughts in the lab using cannabis, known to cause temporary paranoia.

In other words, by getting participants high and freaking them out.

A 2015 study injected volunteers with Tetrahydrocannabinol (THC), the chemical that causes cannabis’ mental effects. The study found that even if participants were aware of the possible mental side effects of THC, the injection made them more paranoid than the placebo group by 20 percent.