Published in The Lancet Child & Adolescent Health, the study aimed to provide robust insights into adolescent mental health trajectories in two different cultural contexts by comparing large samples of 11 to 16-years-olds in London and Tokyo over time.

The two studies are the Tokyo Teen Cohort (TTC) and the Resilience Ethnicity and Adolescent Mental Health (REACH) cohorts from South London. Both groups collected data in the period 2014 to 2020 and at three different timepoints as the teenagers grew older.

Researchers used the Short Mood and Feelings Questionnaire (SMFQ) to measure depressive symptoms.

In both groups there was a difference between teenage boys and girls in the average level of depressive symptoms and this difference widened year-on-year.

In the London sample, the gender difference started slightly earlier (evident by 11–12 years) than in the Tokyo sample where it emerged between 11 and 14 years, and the average rate of change in depressive symptoms per year in London girls was around four times greater than among girls in Tokyo.

By the age of 16, the difference in depressive symptoms between boys and girls in London was around twice as large as in Tokyo. The average level of depressive symptoms in teenage boys in Tokyo declined between the ages of 11 and 16, whereas for boys in London it increased slightly over time, following a similar trajectory to that of teenage girls in Tokyo.

First author, Dr. Gemma Knowles, Lecturer in Epidemiology and Youth Mental Health at IoPPN, King’s College London, said, “Our study suggests that from around early adolescence onward girls have higher levels of depression than boys in both Tokyo and London, but the disparity is larger, starts a bit earlier, and increases at a faster rate in London.

“The trajectory for boys in London closely mirrors the trajectory for girls in Tokyo. These findings suggest we need to rethink and challenge popular narratives around gender differences in mental health—for example, the idea that girls are naturally or inevitably more likely to struggle with their emotional health—and to understand the contexts and conditions that enable teenage girls and young people to thrive.”

Currently the Global Gender Gap Index (GGGI)—an index designed to measure gender equality—ranks the UK 15th out of 149 countries while Japan is ranked 125th. However, insights from young co-researchers from both cities suggest there are social influences at play which are not captured in these indexes.

For example, teenage girls may assume adult roles and responsibilities at a younger age in London and there are lower levels of violence and crime in Tokyo. There are also differences in socioeconomic and political trends between the two places that could contribute to differences, for example, the prolonged period of austerity in the UK.

Dr. Knowles said, “This is a complex area with a multitude of possible factors that may contribute to the difference between girls and boys and between the two places. We need to better understand the social contexts and experiences that give rise to gender inequalities in mental health and the conditions that support teenage girls to thrive.

“It’s also important to understand how these social experiences interplay with young people’s physical development at this age.”

An important part of this study was to investigate the extent to which scores derived from the measure SMFQ can be meaningfully and appropriately compared across genders, ages, and contexts.

While these tests are not perfect, they found no evidence of incomparable measurements, which suggests that any differences in SMFQ scores between groups are due to real differences in depressive symptoms rather than differences in reporting.