Any changes in prevalence of autism must be determined

Editor—Because of the current concern over the rising incidence of autism the BMJ owes its readers better information than a letter based on comments by a focus group of interested parties.¹ Would the journal have published a letter from a focus group of parents who believed that MMR vaccine (measles, mumps, and rubella vaccine) had caused their child's autism? I doubt it.

Rigorous methods are especially important because of a controversial current hypothesis.² Fombonne has argued that despite reported increases in the prevalence of autism in many countries, the true incidence has remained constant. The impression of an increase, he argues, arises only because of increased rates of detection.

There is no hard evidence to support this hypothesis. The only studies that have explored the question have failed to find a “hidden horde” of autistic children. Burd et al conducted a prevalence study of autism in North Dakota.³ They found a prevalence of 3.26 per 10 000 among a cohort of children born between 1967 and 1983. A 12 year follow up survey of the same cohort showed that the original prevalence study had found 98% of the autistic children in the area; only one child had been overlooked.⁴

Nylander and Gillberg screened adult psychiatric outpatients for evidence of undiagnosed autistic spectrum disorders.⁵ This population had not been screened for autism previously. The authors hypothesised that they would find high rates of undiagnosed autism. The screening procedure located 19 adults with autistic spectrum disorders who had not received a prior diagnosis. However, the prevalence in this group was only 2.7 per 10 000, a finding that provides little support for a hidden horde hypothesis. The authors note this point reluctantly, claiming that the observed prevalences “should be regarded as an absolute minimum.”

Many scientists and health professionals are uncomfortable about the data regarding recent increases in rates of autism. A few have developed extravagant theories as an expression of their discomfort. Nevertheless, the simplest interpretation of the record supports the conclusion that the incidence of autism has increased.

To avoid the consequences of complacency the burden of proof should lie with those who seek to dismiss decades of epidemiological research as flawed. Yet we are now offered focus groups as a new research tool. I am surprised that the BMJ would dignify such efforts. Good science demands that we face the real data, no matter how inconvenient the implications may be.