Study analysis · Psychoneuroendocrinology · 2026
Teen hair cortisol dropped over two years—but it barely tracked depression, anxiety, or childhood adversity, except in sex-specific, exploratory subgroups.
In 302 teen twins, the stress hormone stored in hair went down over about two years, and it wasn't a broad marker of mental health—though in girls high hair cortisol went with severe life stress, and in boys with more anxiety.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
In simple terms
This study watched a group of teens over time, measuring stress hormones in their hair and asking about their feelings. It can show that two things happen together, but it cannot prove that one causes the other. For example, if higher hair cortisol is linked to anxiety in boys, we cannot say cortisol causes anxiety or vice versa.
What’s the bottom line?
Scientists measured cortisol in hair samples from 302 teen twins twice, about two years apart. They wanted to see if long-term cortisol relates to stress, anxiety, depression, and tough life events.
How strong is this study?
The study is fairly strong because it followed the same kids over about two years, used careful lab methods, and had a decent number of twins. But it was not a randomized experiment, so other things could explain the links, and some groups were small. So we can trust the patterns somewhat, but they are not proof.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
32 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=302)+15.6/20
- Follow-up+10/10
100 / 100
77 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervals+15/15
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 558 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
This design cannot establish causation — the findings describe an association, not a cause. This is an observational longitudinal cohort study without randomization or a control group. It can demonstrate temporal associations but cannot rule out confounding, reverse causation, or unmeasured variables. The analyses were exploratory, several subgroups were small, and the sample was socioeconomically homogeneous, all of which limit causal inference.
COI Unknown
Could not determine conflict of interest status
No conflicts of interest or funding details are provided in the available excerpt; the article includes a Declaration of Competing Interest and Funding section but their contents are not visible.
The excerpt only includes the article's outline and introduction; the full text of the funding and competing interest statements is not available. No industry affiliations are apparent from author names or affiliations, but this cannot be confirmed.
Key takeaways
- 01
Hair cortisol dropped over ~2 years (Cohen's d=0.34, small-to-moderate).
- 02
It did not differ by sex, twin type, or puberty.
- 03
Overall, hair cortisol was not linked to depression, anxiety, daily stress, or childhood adversity.
- 04
In boys, higher hair cortisol was linked to more general anxiety.
- 05
In girls, 15% of those with the highest hair cortisol had experienced a severe lifetime stressor vs 0% of those with the lowest hair cortisol (p=0.01; absolute difference 15 percentage points).
- 06
This was an observational study, so it cannot prove cause and effect.
- 07
The absolute risk increase was not reported.
- 08
The 15% vs 0% finding is a 15 percentage-point difference in extreme quartiles, but it is based on small groups (about 34–40 girls per quartile) and needs replication.
- 09
Hair cortisol may not be a broad biomarker for all teen stress or mental health.
Surprising findings
- Hair cortisol decreased over ~2 years despite the COVID-19 pandemic, rather than increasing as hypothesized.Chronic stress and lockdowns were expected to raise cumulative cortisol in adolescents.
- HCC was not significantly associated with depression, anxiety, daily stress, ACEs, or bullying overall.Hair cortisol is widely promoted as a biomarker of chronic stress and mental health risk.
- HCC showed poor longitudinal stability and minimal twin-pair concordance, with negative intraclass correlations.A biomarker of 3-month cumulative cortisol should be somewhat stable within a person over time and more similar in identical twins if genes matter.
- Dizygotic twins had slightly higher HCC concordance than monozygotic twins.Identical twins share all genes, so they should be more similar than fraternal twins if cortisol is heritable.
- Higher baseline HCC predicted lower later anxiety on the SCAS.Most people expect higher chronic cortisol to predict more anxiety, not less.
Practical takeaways
Don't rely on a single hair cortisol test as a broad screening tool for teen depression, anxiety, or chronic stress.
This study was in a homogeneous, socioeconomically advantaged Australian twin sample; absolute risk increases were not reported for most outcomes.
medium confidenceIf a teen girl has very high hair cortisol, it may be worth gently exploring severe lifetime stressors—but not as a diagnosis.
The 15% vs 0% finding came from extreme quartiles with small groups (~34–40 girls) and was exploratory.
low confidenceFor teen boys, unexplained anxiety might co-occur with higher cumulative cortisol, but the link was modest and p<0.05 only.
The male anxiety association was not the study's main finding and could be a false positive given many tests.
low confidenceResearchers and clinicians should measure absolute risks and use diverse samples before generalizing hair cortisol findings.
The study's sample was 52% female, 54% monozygotic, and largely high SES; ACEs were parent-reported and likely underreported.
medium confidenceWhy this study matters
Hair cortisol falls during adolescence
Average hair cortisol concentration (HCC) decreased significantly from Session 1 to Session 2 (Cohen's d=0.34, 95% CI 0.19–0.51; small-to-moderate absolute standardized effect). This was a within-person drop over roughly 13–30 months in 302 adolescent twins.
Most people assume stress hormones rise as teens face puberty, school, and COVID. This study found the opposite: cumulative cortisol in hair went down.
Overall, hair cortisol didn't match mental health
HCC was not significantly associated with concurrent or later depression, daily stress, anxiety, adverse childhood experiences (ACEs), or bullying. For ACEs: β=0.20, p=0.61; for bullying: β=0.036, p=0.43. All p>0.05 for main mental health outcomes.
Hair cortisol is often marketed as a chronic stress biomarker. This study suggests it is not a broad mental-health thermometer in healthy teens.
Sex-specific links: girls and severe stress, boys and anxiety
In females, 15% of the highest HCC quartile reported at least one severe lifetime stressful event vs 0% of the lowest quartile (p=0.01; absolute difference 15 percentage points; relative risk not estimable because baseline was 0%). In males, higher HCC at Session 2 was associated with elevated general anxiety (p<0.05). These subgroup analyses were exploratory with small groups (about 34–40 girls per quartile).
The same biomarker pointed to different things in boys and girls: anxiety in boys, severe life stress in girls.
Poor stability and minimal twin concordance
HCC showed poor longitudinal stability within individuals over ~2 years, with negative intraclass correlations, and minimal concordance within twin pairs. Dizygotic twins had slightly higher ICCs than monozygotic twins—opposite of a strong genetic pattern. This suggests non-shared environmental factors matter more than genes for long-term cortisol in this cohort.
If identical twins don't have more similar hair cortisol than fraternal twins, then genes may matter less than daily environment and context.
No effect of sex, puberty, zygosity—or COVID
HCC did not differ significantly by biological sex, twin zygosity, or pubertal stage. The study also expected COVID-19 to raise HCC, but it actually decreased. Only one seasonal contrast was significant: summer vs autumn, d=−0.40, p=0.040, small and likely due to subtropical climate stability.
Many assumed the pandemic would spike teen stress physiology. This cohort from Brisbane showed the opposite, possibly due to adaptation or limited lockdowns.
Higher baseline HCC predicted lower later anxiety—unexpectedly
In linear mixed models, higher HCC at Session 1 was associated with lower anxiety scores on the SCAS at Session 2 (β=−0.92, SE=0.34, p=0.008). This contradicts the idea that higher cumulative cortisol always predicts worse mental health.
It flips the expected direction: more cortisol in hair predicted less anxiety two years later, at least in this sample.
Want the whole report?
Detailed mode opens the full scientific breakdown — every score component, the methodology, conflicts of interest, the evidence analysis behind each claim, and the raw study data.
Overview
What the study found
The study in plain English — the bottom line, every takeaway we extracted, and what to do with them.
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists measured cortisol in hair samples from 302 teen twins twice, about two years apart. They wanted to see if long-term cortisol relates to stress, anxiety, depression, and tough life events.
Research results
Hair cortisol dropped over ~2 years (Cohen's d=0.34, small-to-moderate). It did not differ by sex, twin type, or puberty. Overall, hair cortisol was not linked to depression, anxiety, daily stress, or childhood adversity. In boys, higher hair cortisol was linked to more general anxiety. In girls, 15% of those with the highest hair cortisol had experienced a severe lifetime stressor vs 0% of those with the lowest hair cortisol (p=0.01; absolute difference 15 percentage points).
What this means - more context
This was an observational study, so it cannot prove cause and effect. The absolute risk increase was not reported. The 15% vs 0% finding is a 15 percentage-point difference in extreme quartiles, but it is based on small groups (about 34–40 girls per quartile) and needs replication. Hair cortisol may not be a broad biomarker for all teen stress or mental health.
To examine whether hair cortisol concentration (HCC), a biomarker of cumulative cortisol over ~3 months, is associated with concurrent and later mental health, stress exposure, and adverse childhood experiences in community-dwelling adolescents.
In 302 adolescent twins followed ~2 years, average HCC decreased significantly between sessions (Cohen's d=0.34, 95% CI 0.19–0.51, small-to-moderate). HCC did not differ by sex, twin zygosity, or pubertal stage and was not associated with concurrent depression, anxiety, daily stress, ACEs, or bullying. Sex-specific findings: in males, higher HCC was associated with elevated general anxiety symptoms; in females, higher HCC was associated with severe lifetime stress (15% in highest quartile vs 0% in lowest quartile, p=0.01; absolute difference 15 percentage points). HCC showed poor longitudinal stability and minimal twin-pair concordance.
Methods Used
Longitudinal twin cohort (n=302; 52% female; 54% monozygotic) from Brisbane, Australia, assessed at two sessions 13–30 months apart. Three-cm hair samples quantified cumulative cortisol over ~3 months. Self-report measures included depression (SMFQ), anxiety (SCAS), daily stress (DLSS), ACEs, bullying (GBS), and stressful life events. Linear mixed-effects models and quantile regression examined associations.
Main Finding
Average HCC decreased significantly from Session 1 to Session 2 (Cohen's d=0.34, 95% CI 0.19–0.51). No main effect of sex, zygosity, or pubertal stage on HCC. HCC was not significantly associated with concurrent or later depression, daily stress, ACEs, or bullying. In males, higher HCC at Session 2 was associated with elevated general anxiety (p<0.05). In females, higher HCC was associated with severe lifetime stress: 15% of the highest HCC quartile vs 0% of the lowest quartile reported at least one severe stressful event (p=0.01; absolute difference 15 percentage points). Absolute risk increases were not reported.
Confidence Level
Low to moderate. Strengths include longitudinal twin design, n=302 (large for adolescent HCC twin research), and assay reliability. Limitations include exploratory analyses, small subgroups for sex/ACE analyses, low ACE prevalence, homogeneous socioeconomically advantaged sample, parent-reported ACEs, and no reported absolute risk estimates.
Study Flags
Red Flags
- •Exploratory analyses with small sex-specific and ACE subgroups; findings may reflect low power rather than true null associations.
- •Homogeneous, socioeconomically advantaged sample limits generalizability to higher-risk or more diverse adolescents.
- •Parent-reported ACEs likely underreport adversity, and absolute risk increases were not reported.
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Hair cortisol decreased over ~2 years despite the COVID-19 pandemic, rather than increasing as hypothesized.
Chronic stress and lockdowns were expected to raise cumulative cortisol in adolescents.
Practical Takeaways
Don't rely on a single hair cortisol test as a broad screening tool for teen depression, anxiety, or chronic stress.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 558 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
Human Cohort Study
Subject
Moderate probability
on the GRADE evidence scale
This study watched a group of teens over time, measuring stress hormones in their hair and asking about their feelings. It can show that two things happen together, but it cannot prove that one causes the other. For example, if higher hair cortisol is linked to anxiety in boys, we cannot say cortisol causes anxiety or vice versa.
Strengths
- Longitudinal design with two assessment sessions approximately two years apart.
- Twin design with mixed-effects models to account for family-level clustering.
- Relatively large sample for hair cortisol research in adolescents (n=302).
Weaknesses
- Observational design without randomization or a control group.
- Analyses were treated as exploratory, with limited power for stratified and subgroup analyses.
- Low prevalence of ACEs and severe stressful events reduced sensitivity to detect associations.
Methodology
Evidence Keywords
Statistical Reporting
Not medical advice. For informational purposes only. Always consult a healthcare professional. Terms
Scientists measured cortisol in hair samples from 302 teen twins twice, about two years apart. They wanted to see if long-term cortisol relates to stress, anxiety, depression, and tough life events.
Research results
Hair cortisol dropped over ~2 years (Cohen's d=0.34, small-to-moderate). It did not differ by sex, twin type, or puberty. Overall, hair cortisol was not linked to depression, anxiety, daily stress, or childhood adversity. In boys, higher hair cortisol was linked to more general anxiety. In girls, 15% of those with the highest hair cortisol had experienced a severe lifetime stressor vs 0% of those with the lowest hair cortisol (p=0.01; absolute difference 15 percentage points).
What this means - more context
This was an observational study, so it cannot prove cause and effect. The absolute risk increase was not reported. The 15% vs 0% finding is a 15 percentage-point difference in extreme quartiles, but it is based on small groups (about 34–40 girls per quartile) and needs replication. Hair cortisol may not be a broad biomarker for all teen stress or mental health.
To examine whether hair cortisol concentration (HCC), a biomarker of cumulative cortisol over ~3 months, is associated with concurrent and later mental health, stress exposure, and adverse childhood experiences in community-dwelling adolescents.
In 302 adolescent twins followed ~2 years, average HCC decreased significantly between sessions (Cohen's d=0.34, 95% CI 0.19–0.51, small-to-moderate). HCC did not differ by sex, twin zygosity, or pubertal stage and was not associated with concurrent depression, anxiety, daily stress, ACEs, or bullying. Sex-specific findings: in males, higher HCC was associated with elevated general anxiety symptoms; in females, higher HCC was associated with severe lifetime stress (15% in highest quartile vs 0% in lowest quartile, p=0.01; absolute difference 15 percentage points). HCC showed poor longitudinal stability and minimal twin-pair concordance.
Methods Used
Longitudinal twin cohort (n=302; 52% female; 54% monozygotic) from Brisbane, Australia, assessed at two sessions 13–30 months apart. Three-cm hair samples quantified cumulative cortisol over ~3 months. Self-report measures included depression (SMFQ), anxiety (SCAS), daily stress (DLSS), ACEs, bullying (GBS), and stressful life events. Linear mixed-effects models and quantile regression examined associations.
Main Finding
Average HCC decreased significantly from Session 1 to Session 2 (Cohen's d=0.34, 95% CI 0.19–0.51). No main effect of sex, zygosity, or pubertal stage on HCC. HCC was not significantly associated with concurrent or later depression, daily stress, ACEs, or bullying. In males, higher HCC at Session 2 was associated with elevated general anxiety (p<0.05). In females, higher HCC was associated with severe lifetime stress: 15% of the highest HCC quartile vs 0% of the lowest quartile reported at least one severe stressful event (p=0.01; absolute difference 15 percentage points). Absolute risk increases were not reported.
Confidence Level
Low to moderate. Strengths include longitudinal twin design, n=302 (large for adolescent HCC twin research), and assay reliability. Limitations include exploratory analyses, small subgroups for sex/ACE analyses, low ACE prevalence, homogeneous socioeconomically advantaged sample, parent-reported ACEs, and no reported absolute risk estimates.
Study Flags
Red Flags
- •Exploratory analyses with small sex-specific and ACE subgroups; findings may reflect low power rather than true null associations.
- •Homogeneous, socioeconomically advantaged sample limits generalizability to higher-risk or more diverse adolescents.
- •Parent-reported ACEs likely underreport adversity, and absolute risk increases were not reported.
No biological mechanisms were identified in this study. This may be an epidemiological, observational, or survey-based study that reports associations rather than proposing causal biological pathways.
Surprising Findings
Hair cortisol decreased over ~2 years despite the COVID-19 pandemic, rather than increasing as hypothesized.
Chronic stress and lockdowns were expected to raise cumulative cortisol in adolescents.
Practical Takeaways
Don't rely on a single hair cortisol test as a broad screening tool for teen depression, anxiety, or chronic stress.
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 558 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
Human Cohort Study
Subject
Moderate probability
on the GRADE evidence scale
This study watched a group of teens over time, measuring stress hormones in their hair and asking about their feelings. It can show that two things happen together, but it cannot prove that one causes the other. For example, if higher hair cortisol is linked to anxiety in boys, we cannot say cortisol causes anxiety or vice versa.
Strengths
- Longitudinal design with two assessment sessions approximately two years apart.
- Twin design with mixed-effects models to account for family-level clustering.
- Relatively large sample for hair cortisol research in adolescents (n=302).
Weaknesses
- Observational design without randomization or a control group.
- Analyses were treated as exploratory, with limited power for stratified and subgroup analyses.
- Low prevalence of ACEs and severe stressful events reduced sensitivity to detect associations.
Methodology
Evidence Keywords
Statistical Reporting
Scoring
How strong is this study?
The study is fairly strong because it followed the same kids over about two years, used careful lab methods, and had a decent number of twins. But it was not a randomized experiment, so other things could explain the links, and some groups were small. So we can trust the patterns somewhat, but they are not proof.
40 / 100
- COI disclosure+40/40
- Data availabilitydata not shared
- Code availabilitycode not shared
32 / 100
- Randomizationnot randomized
- Blindingblinding unclear
- Control groupno control group
- Sample size (n=302)+15.6/20
- Follow-up+10/10
100 / 100
77 / 100
- P-values+15/15
- Effect size+20/20
- Confidence intervals+15/15
- Pre-registrationnot pre-registered
Each component is scored out of 100 and then capped by the study design — a case series cannot reach the ceiling a randomised trial can, however well it is reported.
Where it sits
RCT reviewsReviews of RCTs (Meta-analyses)
Max 100Randomized TrialsRandomized Trials
Max 90Reviews of Cohort StudiesReviews of Cohort Studies
Max 85Cohort StudiesCohort Studies
Max 72Reviews of Case-Control StudiesReviews of Case-Control Studies
Max 63Case-Control StudiesCase-Control Studies
Max 58Cross-Sectional & Case SeriesCross-Sectional & Case Series
Max 50Expert OpinionExpert Opinion
Max 558 / 100
Probability of being correct
Groups of people are followed over time to see who develops an outcome. Strong for identifying risk factors and associations, but cannot prove causation as firmly as RCTs.
This design cannot establish causation — the findings describe an association, not a cause. This is an observational longitudinal cohort study without randomization or a control group. It can demonstrate temporal associations but cannot rule out confounding, reverse causation, or unmeasured variables. The analyses were exploratory, several subgroups were small, and the sample was socioeconomically homogeneous, all of which limit causal inference.
COI Unknown
Could not determine conflict of interest status
No conflicts of interest or funding details are provided in the available excerpt; the article includes a Declaration of Competing Interest and Funding section but their contents are not visible.
The excerpt only includes the article's outline and introduction; the full text of the funding and competing interest statements is not available. No industry affiliations are apparent from author names or affiliations, but this cannot be confirmed.
Standing
The people behind it
The researchers who wrote the study this analysis is built on.
Authored by
7 researchersIf this is your work, this is how we attribute it on Fit Body Science. Sabine Finlay is listed as the lead author.