There is still much to be known about the causes of autism, but research has led to some conclusions.

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There are some genetic risk factors that may lead to autism, such as changes in certain genes. If a parent carries one or more of these gene changes, then the child has an increased risk to develop autism. These genetic changes can also appear spontaneously in an early embryo or in the sperm or egg that fuse to create the embryo. The majority of these gene changes increase the risk for the development of autism spectrum disorder; most do not cause autism alone. 

Autism spectrum disorder may also arise from environmental risk factors, including advanced parental age of either parent, pregnancy and birth complications, and pregnancies spaced less than one year apart. However, the increased risk associated with these environmental factors is very small. Environmental and genetic risk factors seem to affect early brain development. They also appear to alter how brain nerve cells, neurons, and regions of the brain communicate with each other.

As with many categories of disability, the figures about the prevalence of autism tend to vary between studies and depend on how the research is being done. However, with so many studies being done, we can determine estimates that are largely agreed upon by various organizations.

In 2018, the World Health Organization reported that the worldwide frequency of occurrence for autism is about 1 in 160 children, or a little over .6 percent. However, this statistic varies across different studies, and it is likely that actual prevalence rates are significantly higher, especially because we do not have much information on autism in many low and middle income countries. We do know this number has increased over time, though. This increase could be due to many factors, some of which include increased global awareness on autism, a more expansive diagnostic criteria, better tools for diagnosing, and improved reporting of autism statistics. 

Though worldwide prevalence still need to further researched, there is much more confidence in more specific autism statistics, as it’s easier to get more information from a smaller population. For example, the prevalence of autism in the United States and in schools is carefully recorded. These numbers still vary based on the criteria being looked at, but the information itself is valid. As of 2018, it is estimated by the Centers for Disease Control and Prevention that the prevalence of autism in the United States is 1 in 59 people or 1.2 percent.  When looking just at the population of school-age students, The National Center for Education Statistics reports that for the 2017-2018 school year, 10.2 percent of students age 3 to 21 served under IDEA were receiving services and support for autism. In terms of total school enrollment, 1.4 percent of all school children in this age range were identified as having autism. Autism is considered a low-incidence disability, but as mentioned previously, these numbers are rising due to various potential factors.

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