Once primarily limited to severely disabled people, autism began to be viewed as a spectrum that included children and adults far less impaired. Along the way, the disorder also became an identity, embraced by college graduates and even by some of the world’s most successful people, like Elon Musk and Bill Gates. Health Secretary Robert F. Kennedy Jr. has called the steep rise in autism cases “an epidemic.” He blames theories of causality that mainstream scientists reject — like vaccines and, more recently, Tylenol — and even instructed the C.D.C. to abandon its longstanding position that vaccines do not cause autism. Today, Azeen Ghorayshi explains what’s really driving the increase in diagnoses.