Attention Deficit-Hyperactivity Disorder and Month of School Enrollment

Used in 2 entries

For each citing entry, the verbatim excerpt and Likelier's calculation notes (how the source's number was converted to the lifetime-probability framing) are shown below. Click through to read the full claim ledger.

1. [1] Youngest in class Risk

   Statistic

   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.

   “"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."”

   Calculation notes

   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.
   

   Independence note: 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.

   Source date: 2018-11-29 · Accessed: 2026-04-19

2. [2] ADHD diagnosis: pursue vs. wait Decision · inaction side

   Statistic

   Among 407,846 children, the rate of ADHD diagnosis was 34% higher among children born in August than among those born in September in states with September 1 kindergarten cutoffs (RR≈1.34)

   “"Among the 407,846 children in our database who were born between 2007 and 2009, there were 36,319 born in August and 35,353 born in September who lived in 1 of 18 states with a September 1 birthday cutoff for kindergarten enrollment. The rate of ADHD diagnosis was 34% higher among the children born in August than among those born in September."”

   Calculation notes

   Layton TJ, Barnett ML, Hicks TR, et al. N Engl J Med 2018;379:2122–2130. n=407,846 children, quasi-experimental design exploiting September 1 kindergarten enrollment cutoffs across 18 states. Establishes the Relative Age Effect (RAE) on ADHD diagnosis: being the youngest in a classroom increases diagnosis probability by ~34% (RR≈1.34). This corrects the previous "up to 1.6 times" figure in caveats prose, which is not supported by this paper. The RAE represents a documented confounder: developmental immaturity misclassified as ADHD in the youngest classroom quintile. Not used in regret rate arithmetic; cited as a diagnostic-accuracy caveat.
   

   Source date: 2018-11-29 · Accessed: 2026-05-10

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HealthDirect

Youngest in class

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