Researchers found that interactions between depressive symptoms — like sadness, fatigue and a lack of interest — are less predictable in teens but become more fixed in adults, which can lead to persistent depression.

The findings highlight the importance of targeting depression at an early age, when symptoms are still changing, experts say.

Depression is a complex condition, characterised by a range of connected symptoms. Current interventions treat overall depression severity and do not consider how symptoms interact and evolve over time.

Scientists at the University of Edinburgh analysed data from more than 35,000 young people to capture how depression symptoms interact throughout adolescence. The study borrowed and applied an understanding of how temperature affects matter from physics.

As temperature rises, particles move more freely and the system becomes less stable, which can be seen as matter changes from solid to liquid to gas.

The research team applied this idea to depression symptoms, using network analysis where symptoms are connected like nodes in a web. From this, they calculated the ‘network temperature’ to capture how fixed or flexible symptom patterns are.

Symptom patterns become more stable across adolescence, with individuals more likely to be persistently depressed or experience no depressive symptoms, while symptoms fluctuate at younger ages.

Experts say the variability seen in teen depression is likely to be influenced by three main factors: puberty and hormones; ongoing brain development; and social and environmental influences.

Researchers also found that among teenagers, depression symptoms stabilise faster in boys than girls, leaving less time for risk or protective factors to have an effect. Symptoms in teenage girls continue to fluctuate over a longer period.

Targeted support for young teenagers while symptoms are flexible and more responsive to treatment could help to prevent persistent depression into adulthood, the research team says.

The findings could also help to explain why some adults — with stable symptoms which are unable to change — experience depression that is resistant to treatment. But experts say further research is needed to explore the theory.

The study, funded by the Medical Research Council and Wellcome Trust, is published in the journal Nature Mental Health.

The research team included scientists from the University of Strathclyde, University College London, Karolinska Institute, and the National University of Singapore.