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1. [1] New England Journal of Medicine (Layton, Barnett, Hicks, Jena 2018) — Attention Deficit-Hyperactivity Disorder and Month of School Enrollment ↗ 1 other entry Рецензована публікація

   Attention Deficit-Hyperactivity Disorder and Month of School Enrollment

   See all 2 Likelier entries citing this source →

   Статистика

   Among 407,846 children born 2007-2009, ADHD diagnosis rate was 85.1 per 10,000 for August-born children vs 63.6 per 10,000 for September-born children in states with a September 1 kindergarten cutoff — a 34% relative increase. ADHD treatment rate was 52.9 vs 40.4 per 10,000 (31% higher). No significant difference existed for other consecutive birth months or in states without a September 1 cutoff.

   Витяг

   “"The rate of ADHD diagnosis among children in states with a September 1 cutoff was 85.1 per 10,000 children among those born in August and 63.6 per 10,000 children among those born in September." ”

   Дані джерела:

   2018-11-29

   Дата звернення

   2026-04-19 · архівна копія

   Розрахунок

   Layton et al. 2018 is the gold-standard natural experiment on relative age and ADHD diagnosis. The study used insurance claims data for 407,846 children born 2007-2009 across 18 states with a September 1 kindergarten cutoff. The rate ratio 85.1/63.6 = 1.337, yielding the 34% excess diagnosis figure. Crucially, the same August-September comparison in states without a September 1 cutoff showed no significant difference, confirming that the effect is driven by relative age within the classroom rather than by birth month per se. The native numerator (85.1) and denominator (10,000) represent the August-born ADHD diagnosis rate. The normalized 0.34 represents the relative excess risk.

   Незалежність

   Independent dataset from Elder and Lubotsky 2009. Layton et al. use commercial insurance claims; Elder and Lubotsky use NHIS and ECLS-K survey data. Both reach concordant conclusions about relative age and ADHD diagnosis using entirely different data sources and methodologies.

2. [2] Quarterly Journal of Economics (Bedard and Dhuey 2006) — The Persistence of Early Childhood Maturity: International Evidence of Long-Run Age Effects Рецензована публікація

   The Persistence of Early Childhood Maturity: International Evidence of Long-Run Age Effects

   Статистика

   Across 19 OECD countries using TIMSS data, the youngest children in each grade cohort scored 4-12 percentile points lower than the oldest children in grade 4, and 2-9 percentile points lower in grade 8. In Canada and the US, youngest-cohort members were less likely to attend university.

   Витяг

   “"The youngest members of each cohort score 4-12 percentiles lower than the oldest members in fourth grade." ”

   Дані джерела:

   2006-11-01

   Дата звернення

   2026-04-19 · архівна копія

   Розрахунок

   Bedard and Dhuey 2006 is the foundational cross-national study on relative age effects in academic performance. The 4-12 percentile gap at grade 4 narrows to 2-9 percentile points by grade 8, demonstrating partial but incomplete fade. The study used TIMSS 1995 and 1999 international mathematics and science data from countries with clear enrollment cutoff dates. The range reflects variation across countries and subjects. This study anchors the academic-performance dimension of the youngest-in-class effect but does not produce a probability figure used in the normalized estimate.

   Незалежність

   Entirely independent from the Layton ADHD data. Different outcome (test scores vs ADHD diagnosis), different data source (TIMSS international assessments vs US insurance claims), different time period.

3. [3] Journal of Health Economics (Elder 2010, building on Elder and Lubotsky 2009) — The Importance of Relative Standards in ADHD Diagnoses: Evidence Based on Exact Birth Dates Рецензована публікація

   The Importance of Relative Standards in ADHD Diagnoses: Evidence Based on Exact Birth Dates

   Статистика

   Children born just before the kindergarten cutoff date had ADHD diagnosis rates 8.4% vs 5.1% for those born just after — a 65% relative increase. They were also more than twice as likely to regularly use methylphenidate (Ritalin). The effect was driven primarily by teacher assessments, not parental assessments.

   Витяг

   “"Roughly 8.4 percent of children born in the month prior to their state's cutoff date for kindergarten eligibility are diagnosed with ADHD, compared to 5.1 percent of children born in the month immediately afterward." ”

   Дані джерела:

   2010-08-01

   Дата звернення

   2026-04-19 · архівна копія

   Розрахунок

   Elder 2010 (expanding Elder and Lubotsky 2009) uses ECLS-K and NHIS data to show that the youngest children in kindergarten cohorts are dramatically more likely to receive ADHD diagnoses and stimulant medication. The 8.4% vs 5.1% comparison yields a 65% relative increase, even larger than the 34% found in Layton et al. 2018. The higher relative effect likely reflects different age ranges and data sources (survey vs claims). The key mechanistic finding is that teachers drive the diagnosis disparity: teacher assessments of ADHD symptoms are strongly correlated with relative age, while parental assessments are only weakly correlated. This suggests teachers are comparing children to classmates who may be up to 11 months older, interpreting normal developmental variation as pathology.

   Незалежність

   Uses ECLS-K and NHIS survey data, independent from the Layton et al. 2018 insurance claims data. Different methodology and era but concordant findings, strengthening the evidence base.