It doesn't. And this is one of the great myths and stereotypes about autism that require dispelling. In years past, autism and mental retardation were believed to be synonymous. But thanks to emerging research by Meredyth Goldberg Edelson and Morton Ann Gernbacher, we are learning that individuals with autism are intellectually competent. Thus it would seem that autism is not an issue of mental inferiority at all; it is an issue of disconnection between the body being able to authentically articulate what the brain is telling it by coordinating the production of intelligible speech or written or typed communication or being able to regulate fine and gross motor skills.
Contrast the autistic experience with that of an individual with cerebral palsy to better understand. On the surface, the person with CP appears to have significant limitations which may include little or no speech and lack of motor coordination to ambulate, eat, dress, or bathe; however, their intelligence is wholly intact!
When in doubt, your mantra should always be "presume intellect." This means that we should communicate to and with the person with autism at their chronological age level with total belief that they're "in there," alert, responsive, listening and learning!