1 Why are more children diagnosed with ADHD and ASD? Erik Parner Sektion for Biostatistics Aarhus University A change in registration/recognition or a new environmental factor causing autism?

2 Follow-up until December 31, /88

3 Other psychiatic disorders – ADHD, Tourette’s syndrome, and obsessive-compulsive disorder (OCD) - has experienced a similar increase in number of diagnosed ( Atladottir et al., ). An earlier diagnosis explains some part of the increase. ( Parner et al, ). The heritability coefficient and sibling recurrence risk are contant over calendar time ( Grønborg et al., ). Change in diagnostic reporting practice can explain a large proportion of the increase in autism prevalence for children born ( Hansen et al, 2014 ). Time trend findings

4 Reporting practice and autism Denmark changed from ICD-8 to ICD-10 in 1994 and added outpatients in the Danish Psychiatric Central Research Register in 1995.

5 A thought experiment Cohort A is a reference cohort in which we, for simplicity, assume that no changes in reporting practices occur; increases in prevalence are observed by age. Cohort B is a later cohort where a change in reporting practices was implemented at age 10 years.

6 An expected prevalence curve B exp for cohort B is computed under the scenario of an increase in prevalence of the disorder over time (calendar effect) but assuming that no change in reporting practices took place.

7 Change in disease diagnostic system explain 33%, inclusion of outpatients in the register 41%, and combined they explain 60% (=( )/( )) of the combined increase in autism prevalence.

8 Quite interestingly, the explained proportions did not depend on length of follow-up. whether the child were born in 1980, or in This within cohort analysis is only possible for individuals born prior to Conclusion Change in diagnostic reporting practice can explain a large proportion of the increase in autism prevalence for children born