Prevalence is an estimate of how common a disease or disorder is in a particular population over a certain period of time. Prevalence is calculated as the total number of cases in a population or the total number of cases at a given time, divided by the number of individuals in the population.
Relevance to autism:
According to the U.S. Centers for Disease Control and Prevention, an average of 1 in 110 children in the U.S. have an autism spectrum disorder. However, prevalence estimates vary between states. New Jersey, for example, has an autism prevalence of 1 in 100 children, the highest rate in the U.S. of the disorder1.
The prevalence of autism increased in the state of California from 0.6 cases per 1,000 births in 1995 to 4.1 cases per 1,000 in 20072. However, the spike in cases occurred first in relatively affluent areas. As awareness of the disorder and access to services became more evenly distributed throughout the population, socio-economic factors such as neighborhood affluence became less likely to influence the chances of a child receiving a diagnosis3.
There have also been reports of autism 'clusters,' with higher-than-average rates among Somali immigrants in Minnesota and Sweden.
One group of researchers found that the prevalence of autism spectrum disorders among Somali children aged 7 to 17 years is nearly four times higher than that of non-Somali children in Stockholm4. In Minneapolis, Somalis account for 6 percent of the city's public school population, but make up 17 percent of early childhood special education students with autism, according to statistics from the Minnesota state department of education.
Global autism prevalence:
Lack of awareness about mental health, poor medical infrastructure and social stigma contribute to the poor understanding of autism prevalence in many countries. This makes it difficult to pin down genetic, environmental and cultural factors that may affect prevalence.
The first prevalence estimates in any country are typically low. This is partly due to the fact that early studies tend to be based on medical records, which are often incomplete and, in some cases, non-existent. The first study of autism prevalence in Oman, for example, found only 114 children in the entire country with the disorder, a prevalence of 1.4 cases per 10,000 children5.
A population-based approach, which screens for the disorder among individuals without a diagnosis, is likely to yield much higher numbers. The first comprehensive autism study in South Korea, for example, found that prevalence of the disorder is 2.65 percent, more than double the number of reported cases in the U.S.6. Three-quarters of the children identified in the study had not previously been diagnosed with autism and were being educated in regular classrooms.
Studies in other nations have found rates ranging from 5 cases per 10,000 people in France to 181 cases per 10,000 in Japan.
Rates within nations vary as well. A 2011 study, for example, found that in Western Australia a child is more likely to be diagnosed with autism if he or she is the first-born child, is born to a woman over age 40, or belongs to a family with higher economic status.
Researchers in 12 countries are collaborating on a systematic review, sponsored by the World Health Organization, of autism prevalence around the globe.