Table 2.

Comparison with selected studies from high-income countries.

Income group		Middle-income country		High-income country
Country		Brazil1	Türkiye2	Germany3	United States4
Sample	ADHD, % or rate	1.8–4.5%	9.27%	3.80%	63.6 per 10,000–90.6 per 10,000
	Description	Children in three urban areas of Brazil (Pelotas, Porto Alegre, and São Paulo)	Children in urban district with high socio-economic status (Kadıköy, Istanbul)	Publicly insured children (about 90% of German children)	Children with employer-provided insurance (excludes Medicaid and uninsured)
	Age range (years)	0–22	5–8	4–14	4–7
	School grades	Grades 1–8	Grades 1–2	Grades 1–8	Grades 1–2
Design	Sampling	Community-based	Community-based	Population-based	Population-based
	Observations	11,991	3696	7.2 million	407,846
	Exposure	Young-for-grade	Young-for-grade	Young-for-grade	Young-for-grade
	Study design	Longitudinal	Cross-sectional	Quasi-experimental	Quasi-experimental
	Data source	Parent report	Teacher report	Insurance database	Insurance database
	Measurement	ADHD diagnosis	ADHD diagnosis	ADHD diagnosis and medication	ADHD diagnosis and medication
Findings	Results	Positive association	Positive association	Positive association	Positive association
	Effect size	Risk ratio for ADHD comparing youngest vs. oldest children between 1.11 and 2.02	Prevalence of ADHD among early starters (<72 months) was 15.9% vs. 6.4% among late starters (78–83 months)	Risk ratio for ADHD comparing youngest vs. oldest children was 1.24	Rate of ADHD was 85.1 per 10,000 among early starters and 63.6 per 10,000 among late starters

Notes: ADHD denotes attention deficit hyperactivity disorder. References: Brazil (Caye et al., 2020)¹, Türkiye (Gökçe et al., 2017)², Germany (Schwandt & Wuppermann, 2016)³, and United States (Layton et al., 2018)⁴.