Almost 78 per cent (52,495 children) have virtually "no risk" across all 16 indicators, the study said. Just over 10 per cent were at high risk of developing a mental illness later in life, the researchers reported in the Australian New Zealand Journal of Psychiatry. They found 6.5 per cent of children fell into the "misconduct risk" group, and 4 per cent were classified as having a "pervasive risk". The "pervasive risk" children showed vulnerabilities across all 16 AEDC areas. "Those are the children that are easily identified in the classroom," said lead author Associate Professor Melissa Green at UNSW's School of Psychiatry.

"Those are the [children whom] teachers say are the troublemakers and the parents say they can't control them," she said. Childhood maltreatment was the strongest predictor of the child being in a high-risk group, the researchers found after cross-referencing the AEDC with records from the NSW departments of health, education, justice and child protection. The children in the pervasive risk group were six times more likely to have been maltreated than children in the no risk group. Children in the high risk groups were also two to four times more likely to have a parent with a mental illness or a parent with a criminal history. There was some association between perinatal factors - including mothers smoking during pregnancy, gestational diabetes, birthing complications and low birth weight - but these predictors paled in comparison with maltreatment, parental mental illness and parental criminal history.

Dr Green stressed the study was not diagnosing or labelling five-year-olds with mental illness and cautioned that not all children identified as high risk would go on to develop a mental disorder. Instead it was looking for markers of risk that could act as early warning signs and opportunities for early interventions in the form of psychosocial support, not medication. "We are trying to get to these at-risk children before the emergence of actual symptoms of mental illness, because once those symptoms emerge the process is already on the way to becoming a disorder," Dr Green said. "We don't want to send anyone down a pathway to definitive outcomes. "There will be resilient children who move along and are OK. It will be interesting to find out which ones became resilient to risk factors and why. What did they have going for them that the others didn't?"

In future the study's method of linking AEDC results to other government data, with parents' consent, could be used to identify children at risk and trial interventions, such as psychosocial programs both at school and at home. One major implication using a national assessment in this way was the opportunity to train teachers to look for the particular constellations of behaviours that indicate a child was at risk and intervene at this critical period of brain development, Dr Green said. Government agencies that provide services to vulnerable families could also benefit from sharing data to help target responses depending on the child's needs,. Paul Robertson, chairman of the faculty of child and adolescent psychiatry at the Royal Australian and New Zealand College of Psychiatrists, said the findings confirmed a substantial minority of children as young as five were already showing signs of significant psychosocial risks for mental disorders. "This is a group of very disadvantaged children and we are not responding to their needs or the needs of their families anywhere near adequately enough," Dr Robertson said.

The issues demanded a whole-of-government approach to improve early and targeted intervention services for at-risk families, he said. "It tends to get siloed into health or mental health or education or welfare. Each of those departments can deal with one part of the puzzle, but you need to bring all those things together in a cohesive way," he said.