RS, 2002. The Epidemiology of Autism in California: A Comprehensive
Pilot Study. Report
to the Legislature on the Principal Findings.
NYT coverage of this study
a 1999 study by public health authorities
had indicated that California was experiencing drastic increases
in the number of autism cases, skeptics had attributed
the increase to greater awareness of the condition and more systematic
and better medical diagnosis, or perhaps simply to the growth of
California's population. To determine whether the increase was real,
and if so, what could be concluded about its origins, the State
Legislature requested that the University of California’s
Medical Investigation of Neurodevelopmental Disorders (M.I.N.D.)
Institute conduct a comprehensive pilot study.
new study, led by Dr.
Robert S. Byrd, section chief of pediatrics at the Univ. of
California at Davis Children's Hospital, issued its report on 17
August 2002. It concludes there is "no evidence that loosening
in diagnostic criteria contributed to an increase in the number
of children with autism, and that, therefore, "some, if not
all, of the observed increase represents a true increase
in cases of autism in California."
earlier report, from the California Dept. of Developmental Services,
had noted from 1987 to 1998 a 273% increase in autism cases from
2,778 to 10,360 reported cases. Their analysis indicated a sharp
rise beginning for children after 1980 (figure below).
new study by M.I.N.D. carefully dissected potential factors that
might have misled the earlier report.
the increase could not have been due solely to increases in California's
population, because the number of children in California increased
far less than the number of cases of autism.
critique of the original study was that the criteria used to diagnose
autism could have loosened, artificially inflating the number
of new cases compared to old. To evaluate this possibility, M.I.N.D.
carried out a detailed and independent analysis of two cohorts
of autistic children, one born 1983-85, the second 1993-95. If
criteria had changed, then this comparison should reveal that
the second cohort included children with less severe autism than
the first. M.I.N.D. found that there was no difference in the
autistic characteristics of the two groups and that therefore
changes in diagnosis criteria could not be responsible for the
third potential source of error for the first report was mis-diagnosis.
Perhaps a greater number of children with autism were present
prior to 1980 but not diagnosed... or conversely that in years
after 1980 children were classified as autistic when they should
not have been. M.I.N.D.'s comparison of the 1983-85 and 1993-95
cohorts revealed that this was not the case.
possible explanation for the increase that was suggested was that
more families with autistic chidren were moving into California,
perhaps attracted by a belief that health services in California
were better for autistic chidren than elsewhere. M.I.N.D.'s analysis
showed that in fact not only were the vast majority of autism
cases of children born in California, the younger cohorts were
more likely to have been born in the state than the older cohorts.
This is the opposite of what would be required for this possible
explanation. Hence differential immigration is not driving the
changing demographics in California were responsible for the increase.
M.I.N.D.'s study rejected this explanation after their detailed
analysis of the 1983-85 and 1993-95 cohorts showed that there
were no large differences in the diagnosed children's demographic
All of the alternative explanations of the increase, —factors
that might have made it seem like there was an increase in autism
rates when there really wasn't— were rejected by M.I.N.D.'s
study. This led to the inescapable conclusion that the rate of autism
in children in California has increased. For whatever reason, children
born in California since 1980 are more likely to develop autism
than those born before.
attempted to assess what factors might be involved in causing the
increase. The rate of increase is far too rapid for it to be based
solely on genetic factors. Changes in gene frequency take generations,
not years or even decades. Interactions between genes and environment,
or environmental factors alone, then become the primary suspects.
did not have information about environmental exposures that would
allow them to test the possible contributions of environmental contaminants.
They did perform one analysis to test a proposed medical explanations
for autism—mercury exposure via vaccination—but their
sample size was too small to provide any sure answers.
Department of Developmental Services. 1999. Changes in the
Population of Persons with Autism and Pervasive Developmental Disorders
in California’s Developmental Services System: 1987 through
1998. A Report to the Legislature March 1, 1999. Sacramento
CA: California Health and Human Services Agency.