Live Birth Bias and Observed Associations between Air Pollution and Autism

Abstract

A recent analysis found that exposure to air pollution in specific pregnancy weeks is negatively associated with risk of autism spectrum disorder (ASD) when mutually adjusted for postnatal air pollution exposure. In this commentary, we describe two possible selection bias processes that may lead to such results, both related to live birth bias, i.e. the inevitable restriction of the analyzed sample to live births. The first mechanism is described using a directed acyclic graph and relates to the chance of live birth being a common consequence of both exposure to air pollution and another risk factor of ASD. The second mechanism involves preferential depletion of fetuses susceptible to ASD in the higher air pollution exposure group. We further discuss the assumptions underlying these processes and their causal structures, their plausibility, and other studies where similar phenomenon may have occurred.