Cohort Effects Explain the Increase in Autism Diagnosis among Children Born from 1992 to 2003 in California
The prevalence of autism in the U.S. has dramatically increased over the last 10 years. Decomposition of autism incidence rates into age, period, and cohort effects provide a nuanced analysis of underlying classes of factors that may be etiologically linked to time trends. We estimate an age-period-cohort effect model for autism incidence and also examine variation in effects by autism severity and comorbidity. Compared to those born in 1992, each successively younger cohort has significantly higher odds of an autism diagnosis than the previous cohort, controlling for age and period effects. This effect was primarily limited to high functioning children with autism and those without comorbid mental retardation. This finding supports the hypothesis that a proportion of the autism trend is due to diagnostic drift as the signs and symptoms of autism spectrum disorders become more well-known. We also document a significant effect of period; regardless of age and birth cohort, those observed in 1995 have 3.75 times the odds of an autism diagnosis compared to those observed in 1994 (95% C.I. 2.4-5.9). Period effects in these data reflect changing diagnostic systems, and contribute to observed increases in autism incidence.
Read our paper: Keyes, K., Susser, E., Cheslack-Postava, K., Fountain, C., Liu, K., and Bearman, P.S. (2011) Cohort Effects Explain the Increase in Autism Diagnosis among Children Born from 1992 to 2003 in California. International Journal of Epidemiology.