Rett syndrome is a pervasive developmental disorder first described by Andreas Rett between 1964 and 1966. The disorder did not receive worldwide recognition until after 1983, when an article by Dr. Bengt Hagberg and associates was published. This disorder is quite rare and mostly affects females. The condition is usually fatal in males, resulting in stillbirth or death shortly after birth.

Rett babies appear normal at birth. The characteristic symptoms of Rett syndrome do not appear until the child is at least five months old and usually before the child reaches two years of age. A key feature is a "backward" developmental progression, as affected children lose previously mastered language skills and normal hand gestures become replaced by stereotyped movements common to autistic individuals.

Rett syndrome is one of five pervasive developmental disorders or autism spectrum disorders. This group of disorders is characterized by severe delays in communication and social development. Historically, pervasive developmental disorders like Rett syndrome were lumped into the same category as mental retardation. However, modern research has determined that the conditions are significantly different in one critically different way: Autistic spectrum disorders have more to do with communication deficits than actual intellectual deficits.

Autistic spectrum (PDD) individuals may seem as if they meet the criteria for mental retardation because when they are tested, their IQ scores are low. However, in this case, low test scores are indicative of the fact that PDD children have profound communication problems and do not understand the language that the test depends upon. PDD children may have intellectual capacity but are unable to demonstrate understanding. In contrast, mentally retarded children lack intellectual capacity or understanding. Although the conditions are distinct, they easily can and do co-occur. An autistic person can also be diagnosed with mental retardation, depending on his mental capacity.