Hallucinations are surprisingly common and varied experiences

By Emma Young

Many mentally well people experience hallucinations. An estimated 6 – 15% of us hear, see, feel or even smell things that aren’t real. But there has been little research into what those hallucinations are like — and how they might differ from those experienced by people with psychosis. Now Mascha M.J. Linszen at Utrecht University and her colleagues report the results of a large study of more than 10,000 people aged 14 to 88. The work, published in Schizophrenia, throws up a few surprises among a host of interesting findings.

The team’s online survey was open to anyone aged over 14 who could understand enough Dutch to complete the questionnaires. In total, 10,448 participants (68.9% female) took part.

The team’s first finding was that hallucinations were common. About 80% reported having hallucinated at least once in their lifetime. About half had hallucinated within the past month and 32% within the past week. Auditory hallucinations were most common, with about 30% of the respondents saying they had experienced them in the past month, followed by visual (21.5%), tactile (about 20%) and olfactory hallucinations (about 17%). Of course, the participants were self-selecting, and not gender-balanced, and people who had hallucinated may well have been more likely to take part. It’s also worth noting that the team did not gather medical histories, so some of the participants may have had a diagnosis of psychosis. For all these reasons, it’s not possible to extrapolate these rates to the general population. But the team’s further exploration of answers given by everyone who had experienced a hallucination within the past month did lead to some unexpected results.

There were “remarkably high” percentages of people reporting characteristics of hallucinations previously associated only with schizophrenia, substance abuse and some neurological disorders. For example, almost half of this group had experienced hallucinations in more than one sense. Also, a large proportion of those who reported auditory hallucinations heard music, something previously regarded as rare.

Perhaps the most striking finding, though, was the range of content described. People reported everything from seeing shadows to hearing children crying, and from smelling food to being slapped. There was also a big spectrum of severity — from fleeting, benign experiences to minutes-long, highly distressing hallucinations. The team argues that this finding fits with the idea that psychotic symptoms fall along a continuum, ranging from isolated, minor experiences affecting people who are mentally well to those suffered by people with psychosis. (Though it’s also worth noting that the continuum idea has been challenged.)

A popular explanation for hallucinations is that, in generating these misperceptions, the brain is jumping to conclusions — it’s relying more on faster top-down predictions of what it’s perceiving than bottom-up data from the senses. According to this theory, hallucinations are more likely to happen when the quality of sense data is poor. This could be down to dim lighting, for example, for a healthy individual, or perhaps a failure of the brain to properly process sense data for someone diagnosed with schizophrenia.

As the researchers note, from an evolutionary perspective, fast top-down predictions are especially useful if they potentially aid survival or reproduction. “This might explain why the majority of the contents in our sample involved ‘warning signs’,” the team writes. For example: hearing threatening voices, sirens or footsteps; seeing shadows or figures approaching; feeling insects crawling; smelling fire, smoke or gas; perceiving social cues, such as telephones ringing, kids crying or names being called; feeling hands on shoulders; even smelling food.

To better understand the causes of various types of hallucinations, they suggest that groups of people who experience a specific type, whether they are mentally well or have a clinical diagnosis, should be studied together.

For now though, the findings do suggest that contrary to some portrayals, hallucinations are not rare, and are not necessarily distressing. This extends similar work which had argued that hearing voices, specifically, is more common than often recognised. “Overall, our findings provide a more nuanced and less severe perspective on hallucinations and other misperceptions, which are typically depicted in a negative way and often pathologized in Western news media,” the team concludes. 

Occurrence and phenomenology of hallucinations in the general population: A large online survey

Emma Young (@EmmaELYoung) is a staff writer at BPS Research Digest