Thimerosal is a mercury-containing preservative used in some vaccines, though not used in the MMR, that was suspected to “cause Autism”. This was primarily due to the wave of concern over mercury exposure in the 1990s, which can indeed cause developmental and health problems. Prior to 2000, mercury levels were indeed over the FDA approved amount, so many studies were conducted to test the association. The final report published in 2004 concluded that the mercury in thimerosal did not cause neurodevelopmental disorders, though thimerosal is no longer used in most childhood vaccines anymore. Autism diagnosis rates did not drop after thimerosal was removed from vaccines, thus it could not be a cause.