“There are so many ways a brain can let you down. Like an expensive car, it’s intricate, but mass-produced.” So wrote Ian McEwan in his short work, Saturday. The British novelist wasn’t pondering autism’s heartbreaking and mysterious symptoms when he wrote those words. But he might have been. Autism’s social, communication, and behavioral challenges reflect some biological package of human brain disorders. The origins and the causal pathways of these disorders remain largely unknown.

This week, an epidemiological study in the Archives of General Psychiatry by Joachim Hallmayer and colleagues helped to clarify some of these relationships, but only to a point. The study suggested that, while genetic factors play a critical role in autism, environmental factors appear to be more important than was previously thought. Unfortunately, as so often happens with complex studies, the work has provided a blank canvas on which people have imposed their own preconceptions and narratives.

IN THE INVESTIGATION of autism, some researchers point to pollution exposure or prenatal insults to maternal or fetal health as potential causes. Genetic factors also play a strong role. To help resolve these various influences, several studies compare the incidence of autism and autism spectrum disorders (ASD) among siblings and twins. If one monozygotic (identical) twin satisfies a diagnosis for autism or for ASD, his twin is extremely likely to satisfy the same diagnosis. Such concordance is much higher among monozygotic twins than among siblings who are not twins or among dizygotic (fraternal) twins, indicating a strong role for genetics. Some heritable disorders such as fragile X syndrome are known to cause autism, which also underscores the genetic link.

Some people cite such findings to argue that autism is an entirely genetic disorder. Given the widespread acceptance of unfounded and harmful theories—such as claims that vaccines or cold parenting styles cause autism—researchers and advocates have obvious motivations to emphasize the genetic story. But this stance has pitfalls of its own. It’s doubtful that any single over-arching “cause of autism” is out there to be found. Autism and related disorders include a bundle of related syndromes that probably reflect diverse biological pathways that arise from multiple genetic vulnerabilities and environmental factors.

This week’s study provided more evidence for that nuanced understanding. Using data from a unique resource—the California Autism Twins Study— Hallmayer and his colleagues showed that autism concordance between monozygotic twins is lower than was previously thought. Much more important, autism concordance between dizygotic twins is much higher than previously thought. Because these twins largely share a common environment yet are far less genetically similar to each other than monozygotic twins would be, their concordance in autism symptoms suggests that these common environmental factors play a more important role than had been imagined.