Autism is a brain development disorder characterized by impaired social interaction and communication, and by restricted and repetitive behavior. These signs all begin before a child is three years old. The autism spectrum disorders (ASD) also include the related conditions Asperger syndrome and PDD-NOS, which have fewer signs and symptoms. Autism has a strong genetic basis, although the genetics of autism are complex and it is unclear whether ASD is explained more by multigene interactions or by rare mutations. In rare cases, autism is strongly associated with agents that cause birth defects. Controversies surround other proposed environmental causes, such as heavy metals, pesticides or childhood vaccines; the vaccine hypotheses are biologically implausible and lack any convincing scientific evidence. The prevalence of ASD is about 6 per 1,000 people, with about four times as many males as females. The number of people known to have autism has increased dramatically since the 1980s, partly due to changes in diagnostic practice; the question of whether actual prevalence has increased is unresolved. Autism affects many parts of the brain; how this occurs is not understood. Parents usually notice signs in the first two years of their child's life. Although early behavioral or cognitive intervention can help children gain self-care, social, and communication skills, there is no known cure. Not many children with autism live independently after reaching adulthood, though some become successful, and an autistic culture has developed, with some seeking a cure and others believing autism should be tolerated as a difference and not treated as a disorder.