<P> The scientific study of the causes of developmental disorders involves many different theories . Some of the major differences between these theories involves whether or not environment disrupts normal development, or if abnormalities are pre-determined . Normal development occurs with a combination of contributions from both the environment and genetics . The theories vary in the part each factor has to play in normal development, thus affecting how the abnormalities are caused . </P> <P> One theory that supports environmental causes of developmental disorders involves stress in early childhood . Researcher and child psychiatrist Bruce D. Perry, M.D., Ph. D, theorizes that developmental disorders can be caused by early childhood traumatization . In his works he compares developmental disorders in traumatized children to adults with post-traumatic stress disorder, linking extreme environmental stress to the cause of developmental difficulties . Other stress theories suggest that even small stresses can accumulate to result in emotional, behavioral, or social disorders in children . </P> <P> A 2017 study tested all 20,000 genes in about 4,300 families with children with rare developmental difficulties in the UK and Ireland in order to identify if these difficulties had a genetic cause. They found 14 new developmental disorders caused by spontaneous genetic mutations not found in either parent (such as a fault in the CDK13 gene). They estimated that about one in 300 children are born with spontaneous genetic mutations associated with rare developmental disorders . </P> <P> The first diagnosed case of ASD was published in 1943 by American psychiatrist Leo Kanner . There is a wide range of cases and severity to ASD so it is very hard to detect the first signs of ASD . A diagnosis of ASD can be made accurately before the child is 3 years old, but the diagnosis of ASD is not commonly confirmed until the child is somewhat older . The age of diagnosis can range from 9 months to 14 years, and the mean age is 4 years old in the USA . On average each case of ASD is tested at three different diagnostic centers before confirmed . Early diagnosis of the disorder can diminish familial stress, speed up referral to special educational programs and influence family planning . </P>

Which mental illness commonly begins to exhibit symptoms in early childhood