Many causes of autism have been proposed, but understanding of the theory of causation of autism and the other autism spectrum disorders (ASD) is incomplete. Research indicates that genetic factors predominate. The heritability of autism, however, is complex, and it is typically unclear which genes are involved. In rare cases, autism is associated with agents that cause birth defects. Many other causes have been proposed, such as childhood immunizations, but numerous epidemiological studies have shown no scientific evidence supporting any link between vaccinations and autism.