Prevalence

Early prevalence estimates proposed a rate of autism in 4 to 5 of 10,000 births (Janzen, 2003). More recent prevalence rates estimated a ratio of 2 in 1000 (Wing & Gould, 1979). Present investigations indicate a much higher rate and have raised alarm about contemporary increases in the number of individuals with ASD

The Centers for Disease Control and Prevention (CDC) recently conducted studies in two metropolitan areas. A study of children from 3 to 10 years old in metropolitan Atlanta, Georgia (Yeargin-Allsopp, et al., 2003), found an ASD rate of 3.4 per 1,000 children (about 1:300), including autistic disorder, Asperger disorder, and PDD-NOS, according to DSM IV criteria (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition; American Psychological Association, 1994). Boys were identified at approximately a rate four times higher than for girls; however, this ratio decreased as cognitive impairment increased. For more information visit the website on the Metropolitan Atlanta Developmental Disabilities Surveillance Program of the CDC.

Another study using the same criteria examined the population of Brick Township, New Jersey (Bertrand, et al., 2001). This investigation found an overall ASD rate of 6.7 per 1,000 children (approximately 1:150). Specifically, 4.0 in 1000 were diagnosed with autistic disorder. Of note, there were 2.2 boys for every girl with autistic disorder, a lower ratio than in previous studies. Approximately two thirds of the children with autistic disorder and half of the children with PDD-NOS were also diagnosed with mental retardation. For further information, visit the CDC’s report.

A report from the Department of Developmental Services (2003) in California indicates similar rapid increases in the rate of individuals with ASD receiving their services. According to their data, the current estimated prevalence rate of autism, excluding other pervasive developmental disorders, in California is 3.1 in 1,000 (approximately 1:320), a rise of 774% between 1970 and 1997. The gender ratio was approximately 4 males to 1 female, similar to previous reports of ratios between 3:1 and 5:1 (Lord & Schopler, 1987; Ritvo & Freeman, 1978; Simpson & Myles, 1998). Forty-four percent of the individuals with autism in California were also diagnosed with mental retardation.

The dramatic rise in the rate of ASD has stirred a debate about the cause. Some theorize that the increase may be due to inconsistency of diagnostic criteria and methods, broadening of definitions to include higher functioning individuals with ASD, increased awareness among practitioners, parents, and physicians, or improved and increased services (Simpson & Myles, 1998; Yeargin-Allsopp, 2002). This debate may also be linked to disagreements about the causes of ASD. Currently, there is no clearly identified single cause of ASD, but these disorders are thought to be linked to factors such as (a) genetic or chromosomal disorders, such as Fragile X syndrome (Rutter, 2000); (b) infections, such as congenital rubella; (c) metabolic disorders; (d) toxins, such as alcohol and lead poisoning; (e) anoxia (i.e., loss of oxygen at birth); or (f) traumatic brain injury (Janzen, 2003).

In a nutshell:

• Current estimates of prevalence of autism spectrum disorders (combined): 1:250 to 1:320
• Approximately 44-67% of children with autistic disorder also have mental retardation
• The ratio is 4 boys to every girl diagnosed with ASD