Although researchers aren't 100% sure, autism is thought to arise from abnormalities in brain structure or function. Right now, it's estimated that about 1 in 150 or 6.6 per 1,000 people are diagnosed with Autism Spectrum Disorder. It's found in far more boys than girls, with a ratio of about 4:1. Because autism and other PDDs seem to run in families, there is likely a genetic component to the disorder as well.