If a kid is hearing voices coming from the walls and is seeing ghosts and aliens in trees, such hallucinations or psychotic symptoms in children may have a genetic cause, a new study has revealed.

Many children have behaviours that can seem like psychosis, like having an imaginary friend. But true psychosis is distressing to children and outside their control. The research team from Boston Children's Hospital in the US said that the study findings make a strong case for "chromosomal microarray testing in any child or adolescent diagnosed with psychosis".

Through the Early Psychosis Investigation Center (EPICenter) at Boston Children's, Joseph Gonzalez-Heydrich and his colleagues genetically tested 137 children and adolescents with what's known as early-onset psychosis, or psychotic symptoms appearing before the age of 18.

Based on their findings, published in the American Journal of Psychiatry, they urge chromosomal microarray testing in any child with psychotic symptoms. More than 70 per cent of the children in the study had begun experiencing psychosis before the age of 13. Twenty-eight per cent met formal criteria for schizophrenia, having persistent and unrelenting symptoms.

All underwent systematic testing for DNA duplications and deletions, together called copy number variants or CNVs - and a surprising 40 per cent tested positive. "CNVs were as common as they are in children with autism, who are often screened for CNVs in the clinic. In many cases, the CNVs identified had also been linked to other psychiatric and neurodevelopmental disorders," the researchers explained.

Families are often relieved to learn that their child's psychotic symptoms have a biological component. "Many parents feel like they are put under the microscope, or are even accused of triggering their child's symptoms. It parallels what happened with autism a generation ago," said Gonzalez-Heydrich.

In some children, psychotic symptoms come and go. Psychosis can appear when a child is under stress, angry, very depressed, or having mood swings. But in children with true schizophrenia, symptoms are persistent and extreme.

The earliest signs of psychotic illness may be general. A child may become withdrawn. Their day-to-day functioning may decline, sometimes dramatically, interfering with school and relationships.

They may have outbursts where they hadn't before. Later, hallucinations and paranoia may take hold, causing the child to see and hear things that aren't there, often things that feel threatening.

"The longer psychosis goes untreated, the harder it is to treat later on. If we can treat it earlier and appropriately, the child will likely do better over their lifetime," said David Glahn, a study co-author.