This site naturesplatform.com/#contamination suggests that the modern sitting toilet contributes to the upwards migration of bacteria from the colon. The Americans with Disabilities Act, effective 1990, specifies that the top of a toilet seat be 17-19 inches above the floor. This is up to 3 inches higher than a standard toilet. The ADA forces many able bodied people to use the taller toilets unnecessarily. Like small businesses that have only one toilet. And may have also caused the taller toilets to be installed where they are not required due to economies of scale.

Now find a chart of autism rates. Find 1990. Remember that correlation is not causation! (I don't really think toilets are the main cause but they may be a contributing factor.)