The study, published in Proceedings of the National Academy of Sciences, identified the first biomarker for major, or clinical, depression. This ‘biological signpost’ could mean boys at greatest risk of depression are treated earlier.

Clinical depression affects one in six people at some point in their lives. Until now, however, doctors have lacked a biomarker for clinical depression, partly because its causes and symptoms are so varied.

According to Professor Ian Goodyer of the University’s Department of Psychiatry who led the study: “Through our research, we now have a very real way of identifying those teenage boys most likely to develop clinical depression. This will help us strategically target preventions and interventions at these individuals and hopefully help reduce their risk of serious episodes of depression and their consequences in adult life.”

The researchers collected spit samples from hundreds of teenagers and measured levels of cortisol in the saliva, as well as self-reported information on symptoms of depression. This they used to divide the teenagers into one of four groups depending on their cortisol levels and symptoms of depression.

After following the group for 12 to 36 months, they were then able to work out which group was most likely to develop clinical depression and other psychiatric disorders.

They found boys with high levels of cortisol and depressive symptoms were 14 times more likely to develop clinical depression than those with neither. In girls, however, this difference was less marked. Girls with high cortisol and depressive symptoms were four times more likely to develop clinical depression than those with neither, suggesting gender differences in how depression develops.

The researchers hope that having an easily measurable biomarker – in this case, raised cortisol plus depressive symptoms – will allow primary care services to identify boys at high risk and consider new public mental health strategies for this part of the population.

“This new biomarker suggests that we may be able to offer a more personalised approach to tackling boys at risk for depression,” said co-author Dr Matthew Owens.

“This could be a much needed way of reducing the number of people suffering from depression, and in particular stemming a risk at a time when there has been an increasing rate of suicide amongst teenage boys and young men.”