Autism is a brain disorder that usually becomes apparent in early childhood. Children and adults with autism (autism does not usually improve over time) have problems with communication, social interactions, and routines or repetitive behaviors.

The prevalence of autism, or the percentage of the population that is affected with autism at any given time, appears to have increased over the past few decades. Studies from the 1980s and early 1990s report autism in four to 10 out of every 10,000 children, whereas more recent studies report autism in 30 to 50 out of every 10,000 children.

The causes of this trend remain a mystery. Some have suggested that it is connected to the measles-mumps-rubella (MMR) vaccine, or to thimerosal, a preservative used in immunizations. Others argue that the increased prevalence only reflects a new vigilance in identifying the disorder and a change in the way autistic children are being diagnosed.

In a study published in the January 2005 Archives of Pediatrics and Adolescent Medicine , scientists used current diagnostic criteria to retrospectively diagnose the incidence , or occurrence of new cases, of autism in Minnesota from the mid-1970s to the mid-1990s. The scientists found that the incidence of autism remained relatively stable until 1988-1991, and then increased dramatically through 1997. This suggests that the increase was due to a change in the identification of autism cases rather than to an actual increase in their number.

About the Study

The researchers studied the medical records of all Olmsted County, Minnesota residents who were 21 years or younger between 1976 and 1997. Olmsted County is unusual in that medical records—including detailed developmental, psychiatric, and neurologic information—are available for 95% of residents. The researchers also had access to school records for all of the children, which included notations about developmental, cognitive or learning problems.

The researchers identified all of the children who had received at least one developmental, psychiatric, or neurological diagnosis between 1976 and 1997. Then, using the detailed medical and school records, the researchers applied the current Diagnostic and Statistical Manual of Mental Disorders Fourth Edition (DSM-IV) criteria to determine how many of those children would have been diagnosed with autism using today’s standards.

The Findings

The number of autism cases identified by the researchers increased from 5.5 per 100,000 children in 1980-1983 to 44.9 cases per 100,000 children in 1995-1997. This was an 8.2-fold increase. The incidence rate remained relatively stable from 1980-1983 to 1988-1991, but then began to increase sharply to 29.4 per 100,000 in 1992-1994 and then to 44.9 per 100,000 in 1995-1997.

The study authors applied the DSM-IV criteria for autism to all of the children in the study. However, they were limited by the fact that they could not examine the children directly and had to rely on medical and school records of symptoms and behavior. Parents, teachers, and physicians may not have been aware of the signs and symptoms of autism in the late 1970s and early 1980s, which may have made the medical and school records—on which the researchers based their diagnoses for this study—incomplete. Less severe cases of autism up through the mid-1980s may have been missed altogether.

How Does This Affect You?

According to this study, the incidence of autism increased by more than eight times between the early 1980s and the mid-1990s. However, the study also suggests that the increased incidence of autism beginning in the late 1980s was probably not a true increase. Rather, it was the likely result of new, broader diagnostic criteria introduced in 1987, as well as increased awareness of the disorder, beginning in the late 1980s.

Even if the increase represented actual new cases, it is highly unlikely they were caused by the MMR vaccine. Minnesota law requires that all children receive the full course of MMR vaccines before they begin school. However, the MMR vaccine has been licensed since 1971, and the incidence of autism did not begin to sharply rise in Olmsted County, Minnesota until the late 1980s.

Children usually begin to exhibit the signs of autism by age three. Parents and physicians should look out for any early indications of the disorder. Though there is no cure for autism, many children benefit from early, individualized treatment.