The study, published in Nature Human Behaviour, analyzed data from 3,171 adolescents participating in the Tokyo Teen Cohort, a population-based longitudinal study in which 46.9 percent of participants were girls. The cohort was assessed at three time points, ages twelve, fourteen, and sixteen, allowing researchers to track how experiences unfolding in early and mid-adolescence shaped mental health outcomes later in the teenage years. At age sixteen, the disparities were stark. Girls showed a prevalence of depressive symptoms of 15.9 percent compared with 8.2 percent among boys, nearly a doubling of risk. Suicidal ideation followed a similar pattern, affecting 29.8 percent of girls versus 18.1 percent of boys, meaning nearly one in three sixteen-year-old girls in the cohort reported thoughts of ending their own life.

What distinguishes this investigation from much of the prior literature on sex differences in adolescent mental health is its method of hypothesis generation. Rather than relying solely on theory or prior statistical associations, the research team coproduced the study with young people themselves, working alongside the Bridging Divides Young Persons Advisory Group. This collaborative process, in which adolescents helped shape the conceptualization and design of the research, suggested loneliness and desire for thinness as the most plausible candidate pathways linking female sex to internalizing problems. The approach reflects a growing movement in child and adolescent mental health research toward genuine participation of young people in setting research priorities, a shift that recent commentaries in leading medical journals have argued is long overdue.

To test whether these candidate pathways actually carried a causal contribution, the researchers employed the mediational g-formula, a statistical technique from the field of causal inference that goes well beyond traditional regression-based mediation analysis. The g-formula, developed for settings with time-varying exposures and multiple mediators, allows investigators to estimate what would happen under hypothetical interventions, for example, if loneliness and desire for thinness at age fourteen were eliminated or set to the levels observed among boys. This counterfactual framing is critical because it treats the mediators not merely as correlated variables but as potential intervention targets, and it accommodates the complex web of confounders and intermediate variables that accumulate across adolescence.

The results were striking. The effect transmitted through loneliness and desire for thinness at age fourteen, relative to the total effect of female sex, was estimated at 34.7 percent for depressive symptoms at age sixteen and 46.3 percent for suicidal ideation at age sixteen. In other words, roughly a third of the sex gap in depression and nearly half of the sex gap in suicidal thoughts in this cohort could be statistically attributed to these two intermediates. The researchers visualized their hypothesized causal structure in a directed acyclic graph, adjusting for confounders measured at age twelve, including family and individual characteristics that could independently shape both the mediators and the outcomes. Analyses were conducted in R, with the parametric g-formula implemented through custom code that the team has made publicly available.

The choice of mediators is theoretically meaningful. Loneliness has emerged in recent years as one of the most robust psychological predictors of adolescent psychopathology, with prior work by members of the same team linking chronic loneliness to subsequent psychotic experiences, depression, anxiety, and diminished well-being. Girls may be disproportionately exposed to social disconnection during the middle teenage years, a period when peer relationships intensify in importance and social comparison becomes pervasive. Desire for thinness, meanwhile, taps into body image concerns that are well documented to intensify among adolescent girls, shaped by cultural ideals, weight teasing, and media exposure. Previous research has associated weight perception and weight-related mistreatment with poorer mental health among adolescents, and the present findings suggest that this internalized pressure may be a genuine engine of the depression and suicidality gap rather than a mere correlate.

The study’s framing of sex as the exposure deserves careful attention. In causal inference methodology, there has been extensive debate about whether characteristics like sex, race, or other social categories can be treated as causes of health outcomes, since they are not manipulable interventions in the conventional sense. The authors navigated this methodological terrain deliberately, situating their analysis within a tradition that interprets such disparities as reflecting the consequences of social positioning rather than immutable biology. By estimating the proportion of the total effect that flows through modifiable intermediates, the analysis implicitly asks a policy-relevant question: how much of the gap could close if the social conditions that girls face were changed? The answer, roughly 35 percent for depressive symptoms and 46 percent for suicidal ideation, suggests that a large fraction of the disparity is potentially addressable.

Several caveats temper the interpretation. The data come from a single cohort in Tokyo, and although the Tokyo Teen Cohort is one of the most comprehensive adolescent studies in the world, the generalizability of the specific proportions to other cultural contexts remains to be established. Residual and unmeasured confounding is an ever-present threat in observational mediation analyses, and the authors acknowledge the broader methodological literature on how such bias can distort causal estimates. The mediators were measured at a single age, fourteen, whereas loneliness and body image concerns fluctuate across development; earlier work from the same cohort has shown that distinct loneliness trajectories carry different risks. Moreover, the analysis identifies statistical pathways consistent with mediation, but it cannot prove that intervening on loneliness or thinness desire would fully close the gap, since other unmeasured pathways may operate in parallel.

Even with these limitations, the implications are considerable. The findings suggest that interventions targeting social connection and body image during early adolescence, before the age-fourteen window examined here, could meaningfully reduce the excess burden of internalizing problems carried by girls. Schools, clinicians, and policymakers now have evidence-based targets that emerged not from adult assumptions but from the lived insights of young people themselves. As the global mental health community confronts what researchers have called a silent crisis in girls’ mental health, this study demonstrates that the mechanisms behind the crisis are identifiable, measurable, and, crucially, modifiable. The work was supported by the Japan Society for the Promotion of Science, the Japan Science and Technology Agency, the Wellcome Trust, and the ESRC Centre for Society and Mental Health at King’s College London, and its analytical code is openly shared to enable replication and extension by other research teams worldwide.

Subject of Research: Mediation of sex differences in adolescent depressive symptoms and suicidal ideation through loneliness and desire for thinness in a longitudinal cohort

Article Title: Loneliness and desire for thinness explain sex differences in adolescent internalizing problems

Article References: Loneliness and desire for thinness explain sex differences in adolescent internalizing problems. (n.d.). https://doi.org/10.1038/s41562-026-02578-2

Image Credits: AI Generated

DOI: 10.1038/s41562-026-02578-2

Keywords: adolescent mental health, loneliness, desire for thinness, depression, suicidal ideation, sex differences, Tokyo Teen Cohort, causal mediation, mediational g-formula, internalizing problems, body image, coproduction