Epidemiology: reading the numbers before the cases

This chapter comes before the cases because every epidemiology section that follows leans on the same few surveys, and a reader should know what those surveys can and cannot say before meeting Maya, Jonah, Luis and the others.

One survey that recurs

The National Comorbidity Survey Replication Adolescent Supplement interviewed 10,123 adolescents aged 13 to 18 in the United States and assigned DSM-IV lifetime diagnoses [53]. Nearly half of them, 49.5%, met criteria for at least one disorder by the time of interview; 22.2% had a disorder with severe impairment or distress. By class, anxiety disorders were the most common (31.9%), then behavior disorders (19.1%), mood disorders (14.3%) and substance use disorders (11.4%). The classes arrive in a developmental order: median onset was 6 years for anxiety, 11 for behavior disorders, 13 for mood disorders and 15 for substance use. About 40% of adolescents with one class of disorder met criteria for another. Girls had higher rates of mood and anxiety disorders, boys of behavior and substance use disorders [53].

Three cautions apply every time this survey is cited. The figures are lifetime, not current; an adolescent who met criteria at 9 and is well at 16 counts. They come from structured interviews in the United States, and the sections that follow show how much rates move with instrument, informant, country and definition. And the survey's report does not cover every condition in this book: psychotic disorders, tic disorders and the obsessive-compulsive-related disorders are absent from its list, so their absence from a comparison is a gap in the source, not a finding.

Most adult disorders start in childhood

The Dunedin birth cohort followed 1,037 New Zealanders with diagnostic assessments at ages 11, 13, 15, 18, 21 and 26, which allows a question the retrospective surveys cannot answer: of adults with a disorder at 26, how many had one as children? The answer was 73.9% before age 18 and 50.0% before age 15 [59]. The proportions were higher still among adults who used treatment (76.5% and 57.5%) and intensive services (77.9% and 60.3%). Adult anxiety disorders and schizophreniform disorder were preceded by a wide array of juvenile diagnoses rather than by the same condition, and across adult disorders 25% to 60% of cases had a history of conduct disorder or oppositional defiant disorder [59]. The World Health Organization put the same point more loosely in a 2014 statement that "some studies show" half of all people who develop mental disorders have first symptoms by age 14, alongside its estimate that depression is the leading cause of illness and disability among 10- to 19-year-olds worldwide and that suicide is the third leading cause of death in that age group [11].

Two readings of the Dunedin figures are both right. Most adult disorder has a childhood history (the follow-back design does not say how often a childhood disorder is followed by an adult one), which justifies the attention this book gives to early presentations. And the continuity is often heterotypic: the child's diagnosis need not be the adult's, which is one reason the chapters describe function rather than resting on a label.

Trend claims and their instruments

Several sections report rising rates. Whether a rise is real depends on what was measured. A repeated school survey in one Wisconsin county found the share of adolescents meeting anxiety screening criteria rose from 34.1% in 2012 to 44.0% in 2018 [41], while a meta-analysis of interview-diagnosed depressive disorders in children under 13 found no association between birth cohort and prevalence across 2004 to 2019 [8]. Both can be true: screening thresholds, self-report and interview diagnosis measure different things, in different age groups. The Surgeon General's advisory on social media reports that eighth and tenth graders averaged 3.5 hours a day on social media as of 2021 and that, in a study of 6,595 adolescents aged 12 to 15, those using it more than three hours a day had double the risk of poor mental health outcomes including depressive and anxiety symptoms [1]. The advisory also reports that majorities of adolescents describe benefits, from feeling more accepted (58%) to feeling connected to friends' lives (80%) [1]. A doubling of risk in an observational study, and a 28.9% rise in the suicide rate among 10- to 17-year-olds in the month after a television series was released [16], are associations from designs that cannot establish causation, as the NIMH release itself says of its study.

Disparities are measured against a reference

Where a section reports that a group has a higher rate, the comparison group and the setting matter. In a health-system cohort of 1,347 transgender and gender-nonconforming youth aged 3 to 17, rates of mental health conditions were 3 to 13 times those of cisgender comparison youth, with depressive disorders diagnosed in 49% of transfeminine and 62% of transmasculine 10- to 17-year-olds [13]. Those are diagnoses in a health system, in a population that had reached care; a poll of 716 LGBTQ youth aged 13 to 24 in which 86% of transgender and nonbinary respondents said policy debates had negatively affected their mental health is self-report about experience, not a diagnostic rate [15]. The two kinds of evidence answer different questions and are cited for different things.

What this chapter cannot establish

A prevalence figure describes a population; it does not tell a clinician whether the child in front of them has the condition. The base rates here shape how a presenting complaint should be read, and no more.

Source status

All figures in this chapter were checked against the source: #53 from the article text; #59 from the published abstract; #1, #11, #13, #15, #16 and #41 from the cited pages or open-access text; #8 from the published abstract.

Figures for this chapter

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