A significant primary cluster is found for every birth cohort in every year. All clusters of sole autism are located within a 50 km by 20 km area of Northern Los Angeles centered on West Hollywood. Within this confined area, there is spatial overlap between the clusters that appear for each year (). Children born in these neighborhoods are at approximately four times greater risk of autism than those born in any other place in California (Column 1, ). Relative to known risk factors, this is comparable to the difference between males and females and is twice the risk estimated for maternal age over 40 (Newschaffer et al. 2007
). It is possible that birth clusters -- clusters defined by residence of the child at birth -- arise from compositional dynamics. Parents at risk for a child with autism may select communities to reside in, and then have children. Increased parental age and being male are known risk factors for autism and could drive neighborhood selection. That is, parents with preferences for male children could select specific kinds of neighborhoods to give birth to their child. There is no evidence that neighborhoods vary by proportion of male births, and therefore there is no evidence that neighborhoods are selective on parents who have gender preferences for their children. Likewise, older parents may have residential preferences different than those of younger parents. This is the case as neighborhoods vary by age of parents. Nonetheless, adjusting for parental age does not affect the location of the clusters as can be seen in , and column 2 of .
Most likely or primary high risk clusters of sole autism
Attributes of clusters before and after adjusting for age.
Most likely or primary high risk clusters of sole autism after adjusting for age
The public health impact of the clusters is measured by their PAF. In our analysis, the PAF ranges between a low of 2.4% in 1996 to a high of 4.4% in 1993 (Column 5, ). The PAF is 2.83% for all years. Thus if the primary clusters we identify were absent, the California sole autism caseload would decrease by close to 3%. Given that the primary clusters are bounded within a 20km × 50km area, and account for less than 1% of all births in California at any time over the observation window, this is a striking contribution.
We identify a set of statistically less likely set of secondary high-risk clusters (). The secondary clusters are located in close proximity to the primary clusters. When all significant clusters (primary and secondary) are grouped together, the PAF ranges from 4.4% in 1993 to 15.1% in 1998 (Column 6, ). The PAF for primary and secondary clusters combined is 11.7% for all years.
Age adjusted secondary clusters of high risk
The etiological conditions in the primary clusters are spatially correlated with those of its neighboring regions, and therefore, secondary clusters are more likely to present themselves at a close proximity to the primary clusters than elsewhere. This is explained as a “bleed off” effect of statistical power from the primary clusters (Kulldorff 1997
). In contrast to the primary cluster, some of the smaller secondary clusters are temporally unstable, and while increased risk is observed, they may be a product of chance.
Secondary clusters are less likely than primary clusters. They are more numerous than the primary clusters and therefore give the appearance of covering a large geographical area. Nevertheless, their proximity to the primary clusters, and the absence of any secondary clusters in other parts of California underscore the statistical likelihood of the primary clusters.
An interesting secondary cluster, for the 1994 birth cohort, is a cluster of 12 cases (1.4 expected) centered on the neighborhood of Northridge (Relative Risk=8.7). In 1994, Northridge was the epicenter of the North Ridge Earthquake, and while tenuous, there is research that points to a relationship between increased maternal stress from large scale natural disasters during pregnancy and autism risk (Kinney et al. 2007
). The other secondary clusters of interest are the most likely clusters of low risk. The most likely secondary clusters of low risk are located in sparsely populated areas near San Diego. This is an area geographically and compositionally distinct from the clusters of high risk. The age adjusted most likely secondary clusters of low risk are located in the sparsely populated desert areas in the vicinity of San Diego for the 1996 to 2001 birth cohorts. In a typical year, a child born in these neighborhoods have a 200% to 300% less risk of being born with sole autism than in the rest of California.
The neighborhoods that are part of the primary clusters can be compared to neighborhoods that are not part of primary clusters. We define the boundary of the primary clusters as the outer boundary of the dark blue region in . An appropriate comparison region is a 3 km geographically contiguous area around the primary cluster. This is represented in .
Choosing an appropriate comparison neighborhood
The comparison region (hereafter, buffer zone) has the same area and urbanicity/population density (~3000 people/sq km) as the primary cluster. The median property value in the cluster ranges from $300,000 to $499,999, while in the buffer zone it ranges from $150,000 to $174,999. In addition, 62% of the cluster population -- compared to 41% of the buffer zone population -- is white. Thus the primary cluster neighborhoods are socioeconomically different from the buffer zones immediately around them. Primary clusters are associated with more autism advocacy organizations (in 1993 and beyond) than expected by chance distribution. Primary clusters are not related at any time to DDS Regional Center catchment areas.