Most people have felt depressed or anxious, even if those feelings have never become debilitating. And how many times have you heard someone say, “I'm a little OCD”? Clearly, people intuitively think that most mental illnesses have a spectrum, ranging from mild to severe. Yet most people do not know what it feels like to hallucinate—to see or hear things that are not really there—or to have delusions, persistent notions that do not match reality. You're psychotic, or you're not, according to conventional wisdom.

Evidence is growing, however, that there may be no clear dividing line. Psychiatrists have long debated whether psychosis exists on a spectrum, and researchers have been investigating the question for more than a decade now. A 2013 meta-analysis, combining much of the existing data, by Jim van Os of Maastricht University in the Netherlands and Richard Linscott of the University of Otago in New Zealand, found the prevalence of hallucinations and delusions in the general population was 7.2 percent—much higher than the 0.4 percent prevalence of schizophrenia diagnoses found in recent studies. Now the most comprehensive epidemiological study of psychotic experiences to date, published in July in JAMA Psychiatry, has given researchers the most detailed picture yet of how many people have these experiences and how frequently. The results strongly imply a spectrum—and suggest that the standard treatment for a psychotic episode might be due for an overhaul.

The researchers, led by John McGrath of the University of Queensland in Australia, analyzed data from the World Health Organization's World Mental Health Surveys, a set of community surveys carried out between 2001 and 2009, involving 31,261 adults in 18 countries. After ruling out experiences caused by drugs or sleep, the researchers determined that 5.8 percent of the respondents had psychotic experiences. Two thirds of these people had had only one type of episode, with hallucinations being four times more common than delusions.

The psychotic experiences were typically rare, with 32 percent of sufferers having only a single episode and another 32 percent having two to five. The other third reported between six and more than 100. Having more than one type of experience was linked to having more in total. These people were not seeking help, and none had been diagnosed with a psychotic disorder. “Most people have only fleeting, sporadic experiences, but there's a subgroup that have a lot, and they're persistent,” McGrath says.

The results suggest psychosis indeed exists on a spectrum, but whether it is distributed in a continuous way across the population remains to be seen. “Is it that we all have a bit of schizophrenia in us, or are there some people who do, and some who don't?” Linscott asks. One complication is that what counts as a hallucination can be a difficult line to draw, and even carefully crafted research surveys can be open to interpretation. “It could be that what we see at the margins are these subtleties due to the language used in the questions,” he says.

The psychotic experiences were slightly more common in women (6.6 percent) than men (5 percent), even though full-fledged schizophrenia is more prevalent in males. In addition, psychotic experiences were more prevalent among people living in middle- and high-income countries (7.2 and 6.8 percent, respectively) than low-income countries (3.2 percent). Being unemployed or unmarried or being from a relatively low-income family was also associated with higher rates of hallucinations and delusions. Socioeconomic and environmental factors such as stress are known risk factors for schizophrenia.

Psychotic experiences are sometimes markers of general psychological distress, McGrath explains: “They pop up in depression, anxiety disorders, a whole range of things.” They also occur in healthy people [see box on opposite page], and ultimately the goal is to discover what determines why some people get a mild dose and are not distressed, whereas others go on to have serious illness. Answering this question could have important ramifications for helping those in distress. Treatment would look very different for a person whose psychotic experiences were linked to depression or anxiety disorders or were the one-time result of acute stress, compared with a person who is showing the first signs of schizophrenia.

The fact that psychosis may exist on a spectrum could also help ease the stigma attached to a schizophrenia diagnosis. That would be a huge boon to people who experience symptoms, no matter how mild or severe.

Benign Hallucinations?

Jenny does not have schizophrenia, but she has hallucinations. “I could feel Mark in the room, standing behind me,” she says of one such experience. “My first love, whom I hadn't seen since I was a teenager, still guiding me, as he had ever since my hallucinations started taking definite shape. I glimpsed him out of the corner of my eye, stroking my spreading wings, reassuring me I'd made the right decision, to leave my old life behind and travel to England to be a journalist.” Jenny, who requested that her real name be withheld for privacy, agreed to talk with me about her hallucinations, which she regards as benign. When she hallucinates, she always sees Mark, and he always offers her advice. He is the part of herself she turns to for guidance.

Jenny believes that her childhood experiences and her mother's mental health issues predisposed her to psychosis—perhaps not surprisingly, because there is a known genetic component. A study last year strongly implicated 108 genetic regions as culprits in schizophrenia. Psychologists have told Jenny that the content of her experiences may be related to a lack of adequate psychological support in childhood, causing her to internalize her own support network. Where mental health is concerned, it seems, nature and nurture are almost always inextricably intertwined.