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.

So what is happening to a person’s brain when they are certain, despite a lack of evidence, that they’re being persecuted?

Mood seems to be a prevalent factor. In cannabis-related paranoia, THC acts on the amygdala, the two almond-shaped clumps of nuclei nestled at the ends of the hippocampus in our temporal lobes. The amygdala processes emotions, but this is also the main area of the brain that induces the fight or flight response. When THC affects the amygdala, it may either temporarily ease anxiety or cause it, depending on general emotional state. If you’re already tense, THC acting on the amygdala can remove your rational understanding that there is no need to worry.

In the article “Cannabis and anxiety: a critical review of the evidence,” published in the journal Human Psychopharmacology, the authors write that “20-30% of users show brief acute anxiety reactions after smoking the drug”—but especially inexperienced cannabis users, teenagers, and people taking high doses. Many experienced an increase in anxiety disorders after the drug effect wore off, though psychiatrists are often divided on whether cannabis worsens long-term anxiety; the paper’s authors believed that, rather than cannabis causing anxiety, people who already experience anxiety issues seek relief in cannabis use (which could weaken other life coping mechanisms).

The paranoia of cannabis and schizophrenia seem to function similarly. “There is no evidence to suggest that the mechanisms are different,” says Dr. Cheryl Corcoran of the Columbia University Medical Center. “Heavy, prolonged cannabis use can lead to psychotic symptoms. … Further, people with schizophrenia can have an exacerbation or relapse of psychotic symptoms when they use cannabis.”  Essentially, cannabis use can cause a signal miscommunication in the brain, which is similar to what might happen in mental illness. “There is a theory that psychosis originates as a consequence of excessive synaptic pruning—synapses are the sites for neurotransmitter release, and enable communication between neurons,” Corcoran says. In this case, neurons can’t send the information that aids in the correct perception of a person’s surroundings and situation. 

While the reason paranoia manifests is not entirely known, it can occur in any person under the right circumstances—even without drug use. The Department of Psychiatry at the University of Kansas found that guilt may fuel paranoid thoughts, possibly via anxiety. In 2008, Daniel Freeman conducted a virtual reality study of 200 mentally healthy people to further establish whether there’s a need for severe mental illness to experience paranoid delusions. The study’s participants first answered questionnaires about their typical experiences with stress and anxiety, and then used virtual reality headsets to ride a virtual London tube train for four minutes alongside neutral, three-dimensional characters reminiscent of early video game CGI. While many of the subjects thought of the characters as friendly, a “substantial minority” said the characters were hostile.

Many of these paranoid subjects were not used to taking the train in the first place. Familiarity and fear response, combined with individual coping mechanisms, personal history, and tendency to worry contributed to whether people responded to stressors with ease or with paranoid explanations. In other words, participants who normally worried more in their daily lives became increasingly worried and suspicious of their surroundings when they no longer felt comfortable or safe. 

In these studies, paranoia seems to be an extension of anxiety, just another human emotion helping us search for meaning in a confusing world. But Corcoran, who has worked extensively with paranoid patients, emphasizes a distinction between psychotic paranoia and the suspicious thoughts of people without severe delusions. “An example I frequently use in presentations is from my treatment of a man with schizophrenia in the 1990’s,” she says. “He believed that aliens had implanted something in his brain and asked for an X-ray, stating that if the X-ray was negative, he would finally consider that this was not true. I was reluctant but he kept asking, so we finally got the X-ray. And it was negative. And he said ... ‘well, maybe what they implanted was made of silicon, and wouldn’t show up on an X-ray.’”

So if your anxieties respond to evidence—if you feel a little better when you get proof that nothing is wrong—you’re probably just experiencing perfectly normal levels of subclinical paranoia. Or you’re high. Or they’re really out to get you.


Welcome to No One’s Watching Week, the time of the year when the readers are away, and your tireless editors have run amok. For this week only, Atlas Obscura, the New Republic, Popular Mechanics, Pacific Standard, the Paris Review, and Mental Floss will be swapping content that is too ​out there​ for any other week in 2015.