A new study published in Nature Human Behaviour has examined why adolescent girls in a Japanese cohort reported higher levels of depressive symptoms and suicidal ideation than boys. Researchers identified loneliness and desire for thinness as two potential pathways that may help explain part of this mental health gap.
The study analysed data from 3,171 adolescents in the Tokyo Teen Cohort, who were assessed at ages 12, 14 and 16. Young people were also involved in the research process to help identify potential pathways for investigation.
By age 16, the study found a notable difference in the prevalence of some internalizing problems.
15.9% of girls reported depressive symptoms, compared with 8.2% of boys. Suicidal ideation was reported by 29.8% of girls and 18.1% of boys in the cohort.
Rather than treating this difference as having one simple explanation, researchers examined experiences at age 14 that could potentially act as pathways between sex differences and mental health outcomes at age 16.
Two factors stood out: loneliness and desire for thinness.
One of the factors identified was loneliness.
Researchers examined loneliness at age 14 as a potential pathway between sex differences and mental health outcomes measured at age 16. Their analysis suggested that loneliness contributed to part of the difference observed in the cohort.
This does not mean that loneliness alone causes depression or suicidal ideation. Instead, the findings suggest that social experiences may be one part of the pathway associated with differences in adolescent mental health.
Feeling disconnected from friends, peers or other people can be an important part of an adolescent's emotional experience. The study highlights loneliness as one area that may deserve closer attention when examining mental health during adolescence.
The second factor was desire for thinness, reflecting concerns about body size and appearance.
Researchers identified this alongside loneliness as a potential pathway that could help explain differences in internalizing problems between girls and boys.
The finding does not establish that concerns about body size directly cause depression or suicidal ideation. Instead, it suggests that body-image experiences may form part of a more complex relationship between adolescent development, social experiences and mental health.
The researchers used a mediational g-formula to estimate the contribution of loneliness and desire for thinness at age 14 to the differences in mental health outcomes observed at age 16.
The analysis estimated that these pathways accounted for 34.7% of the difference in depressive symptoms and 46.3% of the difference in suicidal ideation between girls and boys in the cohort.
These figures should be interpreted carefully. They are mediation estimates, not evidence that loneliness and desire for thinness independently cause these outcomes or that they explain a fixed percentage of mental health problems among girls.
The researchers used these estimates to examine potential pathways contributing to the higher prevalence observed among girls in this particular cohort.
An important feature of the study was the involvement of young people in the research process.
The researchers worked with the Bridging Divides Young Persons Advisory Group and used young people's perspectives to help identify pathways worth investigating. Loneliness and desire for thinness emerged as candidate pathways that were then examined using data from the Tokyo Teen Cohort.
This approach allowed researchers to look beyond simply identifying a difference between girls and boys and investigate experiences that young people considered relevant to adolescent mental health.
The findings from Japan also sit within a broader pattern reported in adolescent mental health research.
WHO's 2021/2022 Health Behaviour in School-aged Children survey included nearly 280,000 young people aged 11, 13 and 15 across 44 countries and regions in Europe, Central Asia and Canada. The report found gender differences in adolescent mental health and well-being, with girls generally reporting poorer outcomes and higher levels of loneliness than boys.
This broader evidence provides useful context, but it does not mean that findings from the Tokyo Teen Cohort automatically apply in exactly the same way to adolescents in every country or social setting.
Together, these findings suggest that understanding adolescent mental health may require attention not only to symptoms, but also to the social and body-image experiences that may occur alongside them.
The study did not test specific treatments or support programmes. However, its findings point towards several areas that parents, schools and youth-support professionals may need to pay attention to.
Early identification: Persistent loneliness, social withdrawal or significant concerns about appearance can be noticed early rather than automatically being dismissed as ordinary teenage behaviour.
Open communication: Teenagers can benefit from having a non-judgmental space where they can talk about friendships, social pressure, body image and emotional difficulties.
Body-image support: Conversations around appearance, weight and dieting can be handled carefully to avoid increasing unhealthy pressure or shame.
Stronger social connection: Supportive relationships with family members, peers, teachers and other trusted adults can help create environments where teenagers feel heard and connected.
Youth-informed support: The study's involvement of young people also highlights the value of considering teenagers' own experiences when developing future prevention and support strategies.
These approaches are not treatments tested by the study. They are areas of support that can be considered alongside professional guidance and further research.
The study does not provide a complete explanation for differences in adolescent mental health between girls and boys.
It was based on participants from the Tokyo Teen Cohort, meaning the findings should be interpreted within that population and not automatically generalised to every country or adolescent group.
Other social, developmental, family, environmental and psychological factors may also contribute to differences in mental health. The study specifically examined loneliness and desire for thinness as potential pathways rather than claiming that they are the only factors involved.
The researchers' findings therefore provide one possible explanation for part of the observed gap, rather than a complete account of why adolescent girls and boys may experience different mental health outcomes.
The findings shift attention from simply asking whether girls experience more mental health difficulties than boys to examining which experiences may contribute to that difference.
By identifying loneliness and desire for thinness as potential pathways, the research provides specific areas for further investigation. It also demonstrates the value of involving young people when researchers are deciding which experiences may be important to study.
For adolescent mental health research, understanding these pathways may help inform future prevention and support strategies while keeping young people's experiences at the centre of the discussion.
Narita, Z. C. et al. “Loneliness and desire for thinness explain sex differences in adolescent internalizing problems.” Nature Human Behaviour, published 15 September 2026.
A related Nature Human Behaviour research briefing, “Why girls have worse mental health than boys,” was published on 22 September 2026.
This article is based on published research and is intended for informational and educational purposes only. Findings from one research cohort should not be interpreted as a diagnosis or as proof that specific factors directly cause mental health conditions. If someone is experiencing significant emotional distress or thoughts of suicide, appropriate professional or emergency support should be sought promptly.
Feeling suicidal or in crisis? Contact a helpline or emergency service immediately.
1. Vandrevala Foundation Helpline:
+91 9999666555 (24x7)
2. Sanjivini (Delhi-based):
011-40769002 (10 am - 5:30 pm)
3. Sneha Foundation (Chennai-based):
044-24640050 (8 am - 10 pm)
4. National Mental Health Helpline: 1800-599-0019
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