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For decades, researchers have looked for links. Most believe the disorder springs from a complex interplay of genetics and environmental factors.

By Emily Baumgaertner Nunn and Azeen Ghorayshi

President Trump and Health Secretary Robert F. Kennedy Jr. released a highly anticipated report Monday on possible causes of autism, an issue that has preoccupied both men for years.

Autism diagnoses in the United States have greatly increased over the last 25 years. Autism experts say the increase is at least partly caused by an increase in awareness and the gradual expansion of the disorder’s definition. But Mr. Trump and Mr. Kennedy have long suggested that vaccines may be playing a role, a theory that has been discredited by dozens of scientific studies.