In this analysis of 10 birth cohorts of California children with autism, we found that socioeconomic factors, most notably parental education, race and maternal immigrant status, influence the age of diagnosis for autism. Socioeconomic status was a stronger predictor than even the severity of symptom expression at the point of diagnosis. Other significant risk factors include birth order, possibly because first-time parents may be less familiar with typical child development and thus slower to recognise the symptoms than more experienced parents, particularly in the early years before autism awareness was widespread. Unlike previous studies, we found no effect for sex.
At the community level, proximity to a child psychiatrist made no difference. Community-level socioeconomic factors had a small impact, although education had a negative effect, net of individual-level education and local financial resources. That is, living in a well-educated neighbourhood does not seem to matter, controlling for parents’ education. Interestingly, the magnitude of community-level effects has diminished over time, as autism has become more prevalent. Some of this declining effect across nearly all risk factors may be due to diminishing variation in the dependent variable over time as age of diagnosis concentrated on age 3 years, leaving less to explain. This is true at both the individual and community levels.
The implications of this study are threefold. First, the factors leading to early diagnosis, although relatively consistent, do shift in their salience somewhat over time, which suggests that study designs pooling disparate cohorts over time might mask important patterns, and that it may be misleading to infer from past cohorts about children born and diagnosed in recent years. Second, as autism prevalence has increased, the age of diagnosis has become less dependent on the community in which a child with autism lives. Finally, we show that there are still risk factors for late diagnosis; since about 1998 these factors are mainly at the individual level.
Although autism awareness is widespread, certain parents, especially those with less education, may be less likely to identify their children's symptoms, or less able to marshal the resources required to obtain an autism diagnosis and services through the DDS system. Systematic differences persist between children in the typical age of diagnosis, as the results in show. Underserved populations are still diagnosed 6 months later than those more advantaged. Although smaller than a decade before, this difference is non-trivial; early childhood is a time of rapid development. If early intervention is important, this disparity in the age of diagnosis has the potential to amplify socioeconomic differences in outcomes later on. These results suggests that physicians and healthcare providers should screen all children aggressively as a routine matter of care, including those whose parents may not have raised concerns. This strategy should continue to exert downward pressure on the age of diagnosis, and may result in the mitigation of durable socioeconomic disparities.
Our data all come from California, which may limit general-isability to other locales. Other research has found significant regional variation in the timing of diagnosis.4
Variation in age at diagnosis should reflect such institutional factors as access to medical care, size and effectiveness of developmental services administration, availability of autism services, the presence of autism advocacy organisations, and visibility and awareness of autism in general. These factors vary by state, as health, disability and educational services are administered mainly at the state level. California's DDS, with its network of regional centres, is especially large and well developed, and has been shown to differ from other states with respect to its pattern of ascertainment of autism versus mental retardation.16
Furthermore, California is a large and geographically and ethnically diverse state, which may account for some differences between our data and those collected at other sites.
However, we believe the particular characteristics of California also provide advantages. In our data, we observe the effects of California DDS improving its system of ascertainment as the caseload rises and the age of diagnosis declines. As other states also improve their systems—particularly in the wake of the federal Combating Autism Act of 2006—we might hope to see less regional variation in the age of diagnosis. The size of the state and the DDS system further allow us to examine these relationships over time and for more recent birth cohorts than other studies. Finally, California is an unusually diverse state, and therefore may provide a glimpse of the country's diverse ethnic future.
The administrative nature of the DDS database is itself a limitation. First, although we do believe that the vast majority of eligible children are enrolled in the DDS, it is voluntary and so probably misses some autism cases in a non-random fashion, such as among underdiagnosed groups. In addition, we are limited by the collection of variables we have assembled from the DDS and the birth registry systems, which collect information for a different purpose than ours. For example, the lack of a measure of cognitive function is lamentable, as it is a factor shown in previous studies to be associated with the age of diagnosis. We remedy when we can with creative uses of available data, and by exploiting geographical variation to incorporate measures of social context. However, the possibility of omitted variable bias remains.
Replication of this type of analysis in other regions would tell us if these patterns are typical, or particular to California and the DDS system. Databases with both geographical and temporal variation, such as the Center for Disease Control and Prevention's autism and developmental disabilities monitoring network will be crucial for understanding patterns of autism diagnosis. Research is also needed to quantify the practical implications of differentials in diagnostic age. A 6-month delay in diagnosis and intervention might have very different meanings for a 3 year old and an 8 year old; however, it is unclear exactly what that difference might mean. Although there is evidence that early identification and intervention is beneficial to children with autism,3 6–11
little is known about how much earlier a diagnosis needs to occur for it to matter, and how the early intervention advantage varies with age.