ADHD diagnoses increase discontinuously by a child's school starting age, with young-for-grade students having much higher ADHD diagnostic rates. Whether these higher rates reflect over-diagnosis or under-diagnosis remains unknown. To decompose this diagnostic discrepancy, we exploit differences in parent and teacher pre-diagnostic assessments within a regression discontinuity strategy based on school starting age. We show that being young-for-grade or male generates over-assessment of symptoms specifically from teacher assessment. However, under-assessments of the oldest students in a grade, especially the oldest females, account for a large part of the observed school starting age assessment gap. We argue that this difference by sex and higher school starting age effects in lower-income schools may exacerbate known gaps in educational attainment by gender and socioeconomic status. Importantly, we fail to find evidence that teachers who receive special education training make such errors.
Keywords: ADHD; gender gaps; human capital; school starting age; teacher training.
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