Childhood has long been championed as a time for make-believe, but recent research has found that another form of unreality – hallucinations – is more common in children than we previously imagined. For years, kids’ accounts of seeing, hearing and experiencing things that weren’t really there were considered to be part of the same invented world – an “overactive imagination”; a “fantasy world”. The Alice in Wonderland approach, perhaps. But as it was recognised that hallucinations can be reliably identified in children, science has begun to look at why these illusory experiences are many times more common during our early years.

Hallucinations often reflect a bizarre, blurry version of our realities and because play is an everyday reality for children, the content can seem similar. Both can contain quirky characters, strange scenarios and inspire curious behaviour. One child described how he saw a wolf in the house, another that he had “Yahoos” living inside him that ate all his medicine. On the surface, these could just as easily be a child’s whimsy, but genuine hallucinations have a very different flavour. “In play and make-believe, children are imagining,” says Elena Garralda, a professor of child and adolescent psychiatry at Imperial College London. “They do not have the actual perceptual experience of seeing and hearing.” Another key difference, notes Garralda, is that “hallucinations feel imposed and children cannot exercise a direct control over them”.

Recent studies have thrown up some surprising statistics about how common they are. One UK study found that almost two-thirds of children reported having at least one “psychotic-like experience” in their lives, a category that also includes unshiftable and unrealistic beliefs and fears. When focusing purely on hallucinations, a review of research found that 17% of 9-12-year-olds have these experiences at any one time. The number roughly halves in teenagers and drops again in adults. Since this type of research tends to focus on experiences that are selected because they can appear in mental health problems in adults, such as hearing voices, which are only a small part of the possible range of hallucinations, these figures are likely to be a low estimate.

It is interesting that hallucinations become less common as we move towards adulthood. Because very young children are more difficult to test and haven’t been studied as widely, it’s not clear whether we start out in a more hallucinatory world, which becomes increasingly stable as we age, or whether middle childhood is a peak time for unreal experiences. For all its reputation for causing emotional mayhem, puberty might be a stabilising force on our perceptions.

At this point, let’s just take a breather – a sanity check if you will – because a lot of people get worried when they think about the possibility of their child hallucinating. These figures don’t mean that if a child is having a hallucination that they are ill or unwell. In the majority of cases, children’s hallucinations disappear within a few days or weeks and are not a cause for concern. Childhood hallucinations are often sparked by life stresses, poor sleep and periods of low mood that fade when the difficult situations do. If the hallucinations are upsetting or persistent, however, it may be time to ask for a professional assessment.

Renaud Jardri has seen many children with hallucinations in his clinical practice and also researches the area as part of his role as a professor of child and adolescent psychiatry at the Lille University school of medicine. The criteria, he says, for judging whether a child needs professional support are whether the hallucinations are “frequent, complex, distressing and cause impairment”. For Jardri, hallucinations that are associated with positive emotions and don’t interfere with the child’s friendships and family life are usually benign.

In rare cases, medical problems can be the cause. Epilepsy can cause hallucinations, as can sleep disorders that affect consciousness and lead to the dream world invading the waking hours. For reasons that aren’t entirely clear, severe psychosis, represented by diagnoses such as schizophrenia, is extremely rare in young children. But when such conditions occur, the hallucinations can be both striking and terrifying. The six-year-old who described having Yahoos living inside him was one of these rare cases; he heard them constantly speak to him, feared being poisoned, believed he could cast spells, smelt “bugs” in the tap water and saw nonexistent trails in the snow. This is a far from the common fear of “monsters under the bed” or isolated hallucinations that fade over time.

Then there are imaginary friends that are not hallucinations but vivid fantasies, which have been the subject of much adult hand-wringing over the years. Because of this, they have been surprisingly well researched and I am delighted to live in a world where there are genuine imaginary-friend scientists, as if Roald Dahl were alive and funding a research institute. It turns out that children with make-believe companions tend to have better social skills and more developed language abilities than kids who lack imaginary buddies. And neither, the research shows, are these illusory companions a compensation for a lack of real friends. They seem to reflect the child’s brain running in overdrive, expending excess energy, delighting in the limits of imagination and playing with the possibilities of the social world.