The evidence that more children and young people are suffering from mental health problems leads to one obvious question: why? While the research was a primarily statistical exercise, it still yielded significant clues about associations, which may also be causes.

What the researchers call “social and family context” clearly matters. Almost a quarter (24%) of people aged five to 19 who have a mental disorder were in a family that have been struggling to function well, the academics found. In comparison, far fewer of them – just 10% – were found to be surrounded by “healthy family functioning”.

Similarly, 28% of troubled youngsters have a parent who has been struggling with mental illness themselves, compared with 9% of children without mental ill health..

Having a parent who has been receiving disability-related income also appeared to be a risk factor, or at least to involve a statistically interesting correlation.

Prof Tamsin Ford, one of the co-authors, said that “a variety of family adversities” was also part of the explanation. That could be parental separation or a financial crisis in their home, explained Ford, a child psychiatrist who focuses on child and adolescent psychiatry at the University of Exeter.

A child who has been socially isolated – who has low levels of social support or does not take part in clubs or organisations in or out of school – “were all associated with the presence of mental disorder”, the researchers found.

With schooling, 8.5% of children with a disorder had been excluded compared with just 0.8% of those who have no mental ailment at all. And while 6.8% of troubled children had been excluded from school, that compared with only 0.5% of all pupils.

The age group for whom it was hardest to pinpoint what caused their mental ill-health was children aged between two and four. Among these preschoolers, one in 19 (5.5%) had a mental disorder, most of which were behavioural disorders (2.5%) such as oppositional defiant disorder (1.9%).

Little-known outside psychiatric circles, these disorders involve extreme temper tantrums and severe disobedience which have been judged to be developmentally inappropriate, severe and persistent, and to cause distress or impair the child’s normal functioning. This new study was the first of the three editions that have been published since 1999 to investigate this age group, so there were no other figures with which to compare the 5.5% finding.

The picture with them has also been complicated by the fact that some of the mental illness in these children was autistic spectrum disorder (1.4%), which is a neurodevelopmental condition, and also that the 5.5% included problems with sleeping, feeding and toileting. There was also the fact that anxiety and depression, the likely causes of which were easier to identify, have been absent in preschoolers.

So, while these two-, three- and four-year-olds were not simply displaying “the terrible twos”, and have been displaying symptoms which psychiatrists could identify from the diagnostic manual that guides their decisions, explaining why many of these children present as they did was not easy.

Those young people aged between 11 and 16 with a disorder were much more likely than their mentally untroubled peers to have taken illicit drugs, drunk alcohol or tried a cigarette.

Among these children, the use of social media, widely blamed for causing much of the epidemic of mental illness in young people, was an important possible explanation too. While, for example, 29.4% of those aged 11 to 19 with a disorder spent more than four hours a day on social media, just 12% of those displaying no symptoms did the same thing.

Those who have a disorder were much more likely to compare themselves with others on social media and to say that “likes, comments and shares impact my mood”. They were also likelier to spend more time on social media than they meant to – and to admit that they could not be honest about their feelings.