Objective To investigate empirically the possibility of an environmental trigger for autism among genetically vulnerable children that is positively associated with precipitation. Design We used regression analysis to investigate autism prevalence rates and counts first in relation to mean annual county-level precipitation and then to the amount of precipitation a birth cohort was exposed to when younger than 3 years, controlling for time trend, population size, per capita income, and demographic characteristics. In some models, we included county fixed-effects rather than a full set of covariates. Setting Counties in California, Oregon, and Washington. Participants Children born in California, Oregon, and Washington between 1987 and 1999. Main Exposure County-level precipitation. Main Outcome Measures County-level autism prevalence rates and counts. Results County-level autism prevalence rates and counts among school-aged children were positively associated with a county's mean annual precipitation. Also, the amount of precipitation a birth cohort was exposed to when younger than 3 years was positively associated with subsequent autism prevalence rates and counts in Oregon counties and California counties with a regional developmental services center. Conclusions These results are consistent with the existence of an environmental trigger for autism among genetically vulnerable children that is positively associated with precipitation. Further studies focused on establishing whether such a trigger exists and identifying the specific trigger are warranted.
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