Final answer:
Atkins v. Virginia caused the 1984 Insanity Reform Act by ruling that executions of people with intellectual disabilities are 'cruel and unusual punishments'.
Step-by-step explanation:
The case that caused the 1984 Insanity Reform Act was Atkins v. Virginia. It was a landmark case in the United States Supreme Court. In this case, the Supreme Court ruled that executions of people with intellectual disabilities are 'cruel and unusual punishments' prohibited by the Eighth Amendment. This ruling led to the Insanity Reform Act, which changed the way insanity defenses were utilized in criminal cases.