An American man who was the first person to be diagnosed with autism died last week in his native state of Mississippi at the age of 89, his family said.

In 1943, at the age of 10, Donald Triplett was diagnosed with having a developmental disability called autism in what was referred to as "Case 1."

Known as "Donald T." in scientific literature, Triplett became a symbol for people with autism seeking fulfillment, becoming the subject of multiple interviews, a documentary and a book.

When he was a child, Triplett did not respond to requests from his parents, was not interested in interacting with other children, but was capable of memorising large amounts of information and performing complex mathematical calculations in his head.

Distraught over his behavior, his parents wrote a 22-page letter to a child psychiatrist, detailing their son's behavior. That document became key in the further study and documentation of the disorder.

Despite his diagnosis, then considered to be a severe handicap, Triplet went to university and led an independent life, working at a local bank for more than 60 years.

He died June 15 at his home in the small city of Forest.

His parents "recognized early in Don's life that he was special in many ways, and they worked diligently to equip him with the means to achieve a happy and productive life," read Triplett's obituary.

In 2020, one in 36 children was diagnosed with an autistic disorder in the United States, according to an estimate from the Centers for Disease Control and Prevention.