Isabelle Rapin, a Swiss-born child neurologist who helped establish autism’s biological underpinnings and advanced the idea that autism was part of a broad spectrum of disorders, died on May 24 in Rhinebeck, N.Y. She was 89.

The cause was pneumonia, said her daughter Anne Louise Oaklander, who is also a neurologist.

“Calling her one of the founding mothers of autism is very appropriate,” said Dr. Thomas Frazier II, a clinical psychologist and chief science officer of Autism Speaks, an advocacy group for people with autism and their families. “With the gravity she carried, she moved us into a modern understanding of autism.”

Dr. Rapin (pronounced RAP-in) taught at the Albert Einstein College of Medicine in the Bronx and over a half-century there built a reputation for rigorous scholarship. She retired in 2012 but continued working at her office and writing journal papers. The neurologist Oliver Sacks, a close friend and colleague, called her his “scientific conscience.”

In his autobiography, “On the Move: A Life” (2015), Dr. Sacks wrote: “Isabelle would never permit me, any more than she permitted herself, any loose, exaggerated, uncorroborated statements. ‘Give me the evidence,’ she always says.”