Complex questions generally have complex answers — such as the question of what causes autism. "I think we found an answer to autism," Trump said. "Diagnosis of autism has always been the biggest challenge in autism research, because we don't have a biological marker of autism," Bird said. Over the past two decades, scientists have carried out many large-scale and rigorous studies to determine whether any aspect of vaccination could cause autism. The false claim that vaccines cause autism originally came from a paper published in 1998 suggesting a link between the measles, mumps and rubella (MMR) vaccine and problems with brain development.