Researchers have defined pervasive developmental disorder as:
"The term "pervasive developmental disorders," also called PDDs, refers to a group of conditions that involve delays in the development of many basic skills. Most notable among them are the ability to socialize with others, to communicate, and to use imagination. Children with these conditions often are confused in their thinking and generally have problems understanding the world around them."
They are referred to as "developmental" disorders because they usually occur when a child is around three years old.
There are five types of pervasive developmental disorders: