A large group of developmental disabilities which are neurological disorders, usually of unknown cause. Characteristics of individuals with PDD include reduced abilities to understand language and communicate normally, reduced ability to socially interact with others in a normal manner, and limited variety in activities and interests. Individuals will also often respond in unusual ways to sensations, engage in repetitive activities, and resist changes to the environment or to daily routines. Types of PDD include autism, Retts Syndrome, Hellers Syndrome, and Aspergers Syndrome, but often if the individual does not have symptoms which fit a particular syndrome s/he will simply be diagnosed with "PDD-NOS" (pervasive development disorder - not otherwise specified).