How did loneliness and body image come into focus?

The researchers did not decide on their candidate factors alone. The study design included what the authors describe as "coproduction with young people" — a collaborative process in which adolescents themselves suggested loneliness and desire for thinness as plausible pathways that might explain the sex gap.

This is where the study's timing matters. Rather than simply comparing girls and boys at a single point in time, the researchers used a statistical technique called the mediational g-formula — a method for estimating how much of an overall effect passes through specific intermediate factors. They asked whether loneliness and the desire to be thin, as reported by participants at age 14, could statistically account for mental health outcomes measured two years later, at 16.

The answer: to a considerable degree. The authors wrote that "the effect through these factors at age 14 relative to the total effect of female sex was 34.7% for depressive symptoms and 46.3% for suicidal ideation at age 16."

Notably, these are not clinical diagnoses of an eating disorder or documented social isolation. They are self-reported feelings — a sense of loneliness and a wish to be thinner — that young people themselves can recognize and describe. That, the researchers suggest, is precisely what makes them promising intervention targets.

Why do these findings matter?

Past research has consistently shown that girls are more likely than boys to experience internalizing problems, and — as Narita, Knowles and colleagues note — this disparity has widened in recent decades. Yet the underlying mechanisms have remained understudied.

"Girls are at higher risk of internalizing problems than boys," the authors wrote. "This disparity has widened in recent decades, but the underlying mechanisms remain understudied."

By quantifying how much of the gap runs through loneliness and body-image concerns, the study gives policymakers, schools and clinicians concrete anchors for support programs. As the researchers put it, the findings "highlight potential targets for interventions and policies informed by young people" — meaning supports designed around what adolescents themselves identify as sources of distress.

What are the study's limitations?

The findings come from one cohort in one country — Japan — and may not generalize perfectly to adolescents elsewhere. The percentages, while substantial, also leave most of the sex difference unexplained: 65.3% of the depression gap and 53.7% of the suicidal ideation gap passed through other pathways the study did not capture. The researchers themselves frame their results as initial insights that could inform future research into additional contributing factors.

The measurements also rely on self-report at three assessment points, at ages 12, 14 and 16, which captures adolescence broadly but not every possible influence on mental health between assessments.

Still, the core message is practical. Two socially shaped experiences — feeling lonely and wanting to be thinner — emerge from a large, prospectively followed cohort as major statistical contributors to one of the most consistent gender patterns in adolescent mental health. Because both can be addressed through social support, school environments and body-image interventions, the study suggests concrete places to start.

The paper, "Loneliness and desire for thinness explain sex differences in adolescent internalizing problems," by Zui C. Narita et al., appears in Nature Human Behaviour (2026), DOI: 10.1038/s41562-026-02578-2.