Cohort effects explain the increase in autism diagnosis among children born from 1992 to 2003 in California

Abstract

Background: The incidence and prevalence of autism have dramatically increased over the last 20 years. Decomposition of autism incidence rates into age, period and cohort effects disentangle underlying domains of causal factors linked to time trends. We estimate an age-period-cohort effect model for autism diagnostic incidence overall and by level of functioning.

Methods: Data are drawn from sequential cohorts of all 6 501 262 individuals born in California from 1992 to 2003. Autism diagnoses from 1994 to 2005 were ascertained from the California Department of Development Services Client Development and Evaluation Report.

Results: Compared with 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. For example, individuals born in 2003 have 16.6 times the odds of an autism diagnosis compared with those born in 1992 [95% confidence interval (CI) 7.8-35.3]. The cohort effect observed in these data is stronger for high than for low-functioning children with an autism diagnosis.

Discussion: Autism incidence in California exhibits a robust and linear positive cohort effect that is stronger among high-functioning children with an autism diagnosis. This finding indicates that the primary drivers of the increases in autism diagnoses must be factors that: (i) have increased linearly year-to-year; (ii) aggregate in birth cohorts; and (iii) are stronger among children with higher levels of functioning.

Figure 1

Incidence of autism diagnosis by age, time period and birth cohort in California among those born 1992–2003

Figure 2

Age-specific incidence of autism diagnosis by birth cohort in California among those born 1992–2003

Figure 3

Predicted diagnosis of autism per 1000 person-years by age, period and cohort. Predicted incidence based on age-period-cohort model shown in Table 1

Figure 4

Cohort effects in autism diagnosis in California from 1994 to 2005 by child's functioning (functioning defined by a global index on two dimensions relevant to autism: social interaction, and communication and language, both recorded on the CDER diagnostic and evaluation instrument at the time of intake. Those above the 80th percentile were considered high-functioning children with autism, those in the 20th to 79th percentile were considered mid-functioning, and those below the 20th percentile were considered low-functioning children with autism) at the time of diagnosis