They looked at records on all 669,995 children born in Denmark from 1990 through 1999. The primary study outcome was the cumulative incidence proportions of each of the targeted disorders:
- Autism spectrum disorders
- Childhood autism (a subset of autism spectrum disorders)
- Tourette's syndrome
- Hyperkinetic disorder
- Obsessive-compulsive disorder.
The data were stratified by age and year of birth for each disorder, grouped into birth cohorts spanning two years.
The investigators identified 4,376 children who received a total of 4,637 diagnoses.
The diagnoses of hyperkinetic disorder, Tourette's syndrome and autism spectrum disorder all showed statistically significant tests for trend.