In the 1950s, many psychoanalysts who followed the Freudian belief system claimed that the condition of autism stemmed from parental withdrawal from the newborn. The mother is primarily held responsible for the child's condition. The name they chose for these dysfunctional mothers was “refrigerator mother,” because of their cold nature. These women were told that they needed to pay more attention to their children or that they were disciplining them too harshly. However, some theories have been examined in recent years. Like family genetics that are passed down from parent to child and there can also be environmental reasons. No one knows exactly how the environment caused ADS; Some hypothesize that vaccines are responsible for the sudden onset of AMS. In particular, the measles vaccine is said to cause autism by damaging the stomach lining. Thimerosal is a preservative found in vaccines, contains mercury and has been linked to this disease. It has also been hypothesized that receiving too many vaccines at once damages the immune system, causing autism. Family genetics research studies have been carried out to identify the number of genes associated with ADS. These studies found irregularities in parts of the brain in people suspected of having SMA. There is also a link to low levels of serotonin or other neurotransmitters in the brain, necessary for maintaining physical activity.