The U.S. Centers for Disease Control found a 9 percentage point increase in the prevalence of confirmed dental fluorosis in a 1999-2002 study of American children and adolescents than was found in a similar survey from 1986-1987 (from 22.8% in 1986-1987 to 32% in 1999-2002). In addition, the survey provides further evidence that African Americans suffer from higher rates of fluorosis than Caucasian Americans. The condition is more prevalent in rural areas where drinking water is derived from shallow wells or hand pumps. It is also more likely to occur in areas where the drinking water has a fluoride content greater than 1 ppm (part per million). If the water supply is fluoridated at the level of 1 ppm, one must consume one litre of water in order to take in 1 mg of fluoride. It is thus improbable a person will receive more than the tolerable upper limit from consuming optimally fluoridated water alone. Fluoride consumption can exceed the tolerable upper limit when someone drinks a lot of fluoride-containing water in combination with other fluoride sources, such as swallowing fluoridated toothpaste, consuming food with a high fluoride content, or consuming fluoride supplements. The use of fluoride supplements as a prevention for tooth decay is rare in areas with water fluoridation, but was recommended by many dentists in the UK until the early 1990s.
Notably, Hayden admits that some children put in this category had no apparent incident to react to, but they were included because of their symptoms. Passive-aggressive mutism: silence is used as a display of hostility, connected to antisocial behavior. Some of the children in her study had reportedly not been mute until age 9–12.The Diagnostic and Statistical Manual of Mental Disorders (DSM), first published in 1952, first included Elective Mutism in its third edition, published in 1980. Elective mutism was described as "a continuous refusal to speak in almost all social situations" despite normal ability to speak. While "excessive shyness" and other anxiety-related traits were listed as associated features, predisposing factors included "maternal overprotection", intellectual disability, and trauma. Elective mutism in the third edition revised (DSM III-R) is described similarly as in the third edition except for specifying that the disorder is not related to social anxiety disorder. In 1994, the fourth edition of the DSM reflected the name change to selective mutism and redefined the disorder. Cultural references
Though elective mutism is no longer recognized by most psychiatrists, it is a popular character element or plot point in stories and movies. Many characters choose to stop speaking, for various reasons. Even more commonly, there are also characters who stop speaking after a traumatic incident. In both these cases, often, and almost always in the second, the character is silent in all situations. This is therefore not selective mutism, and anxiety is very rarely involved. Selective mutism itself is almost nonexistent in pop culture.