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Autism: The Whys of Increased Prevalence

Recent studies indicate an increase in autism prevalence. Columbia University sociology professor Dr. Peter Bearman joins NIMH Director Thomas Insel in this timely  discussion.

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Introduction: Welcome to “Speaking of Science”, the National Institute of Mental Health presents a series of conversations with innovative researchers working in a wide range of disciplines to pave the way for the prevention, recovery, and cure of mental illness.  

In today’s “Speaking of Science” vod cast, Dr. Peter Bearman is the professor of Sociology at the College of Arts and Sciences at Columbia University. He also serves as co- director of the Robert Wood Johnson Foundation Health and Society Scholars program at Columbia. Recently, Dr. Bearman came to the National Institute of Mental Health to talk about the focus of his work, autism prevalence. NIMH’s Director, Dr. Thomas Insel, sat down with Dr. Bearman to discuss autism research and began by referencing recent studies that indicate an increase in autism prevalence.

Dr. Insel:  So, as you look at this that question that everyone is asking is when they see the numbers now from the CDC where it’s gone from 1 in 1500 to 1 in 150 and apparently here in the fall of 2009 the figure that’s emerging is closer to 1 in 100. Meaning, that even since 2002 there has been a very profound increase in the number of children being diagnosed with autism.

Dr. Bearman: And Autism Spectrum Disorders.

Dr. Insel:  Autism Spectrum Disorders?

Dr. Bearman: Well that’s, an important distinction. I mean obviously there’s a profound increase in Autism and Autism Spectrum Disorders.

Dr. Insel: Is there an increase in the number of children with the disorder or with the Autism Spectrum Disorder or does this largely reflect the change in the way the diagnosis is used or some sort of increase in ascertainment?

Dr. Bearman: Well, I think that’s the big million dollar question. Our work which arises from California can, show that changes in diagnostic processing and diagnostic criteria I would say the period from 1992 to 2005 the changes in diagnostic criteria over that period that operate on the border between autism and mental retardation can be associated with about a quarter of the increase prevalence. Over that same period there has been a really fundamental change in the ascertainment, you can see that in lots of ways, but the most obvious way to see the changes in ascertainment, is to see that the social economic status gradient that used to be present for autism, the fact that children living and residing in wealthy communities are more likely to get a diagnosis, and that gradient largely disappears.

Dr. Insel: What seems important Peter in the way you’ve done this rather than you answering the question to say it’s increase, not increased your answering the question by what proportion of increase can be explained by separate factors because everybody’s pointing to changes in diagnosis, changes in ascertainment the way in which services may affect the use of the diagnosis. So what everybody really wants to know at the end of all this, is that actually are more children affected with the disorder or will 100 percent of this increase in prevalence be explained by these other factors?

Dr. Bearman: Our strategy is to try to decompose this increase into its constituent elements. Some component of that is increased ascertainment, some component of that is diagnostic change in diagnostic criteria, some component of that arises from already known risk factors, such as increases in parental age are associated with greater probability of genome mutations that could lead to copy errors that are associated with neurodevelopmental disorders. So social demographic changes that are affecting all western countries, the United States, and also California, can express themselves in increased incidence of autism on top of diagnostic ascertainment dynamics. I think the trick to figuring out how to decompose this increase into its constituent elements is to pay attention to the two dimensions that are important. The first dimension is temporal just the fact of temporal change, we are in the period of increased prevalence and if we’re in a period of increased prevalence and at the same time for example there’s also an increase in older parents. The risk associated with older parents will naturally appear to be greater now than it was a decade ago. So paying attention to temporal heterogenic is important. The other part of our work I think that’s  the most exciting is to pay attention to the spacial heterogenic and the fact that we can observe very strong,  very distinct, very stable clusters of increased risks for autism at very fine spacial resolution. For example, in California, there’s a very clear cluster in about 20 kilometers by 50 kilometers in which the relative risk for autism not, Autism Spectrum Disorders but autism itself is significantly higher over every year of observation that we make than any other place in California. That invites a couple of considerations, first, it invites the recognition that if you observe local spacial clustering whatever causes some components of the increased prevalence in autism it is not a global treatment. Secondly, it invites us to ask, well is there something in that local area that is driving an increased prevalence that could be a shared toxicological environment, it could be a virus that moves through and spreads from person to person and affects children in utero. Or it could be a piece of an ascertainment process which would be the diffusion of information from parent to parent as they learn how to recognize some symptoms for autism which have no biological markers.

Dr. Insel: From what you know now when you add all of those together how much of the increase can you explain?

Dr. Bearman: Well that’s a complicated question, but I think we can pretty uniquely associate about a quarter of the increase from the birth cohorts from 1992 to 2001 which is a lot, to diagnostic change on the border between diagnosis and mental retardation in autism. I think we can associate about 16 percent of the increase on the other border between autism and other neurodevelopmental disorders on the spectrum: Asperger’s, PPDNOS etc. And those are largely non over-lapping components of increase, so that’s about 40 percent. I think the spacial clustering itself adds another few percent. I would say I am confident that 40 percent of the increase I think I know what caused that. That leaves a lot of increase left, 50 percent is a lot to look for still.

Dr. Insel: Any ideas about what’s driving that other 50 percent?

Dr. Bearman: Well, some is genetic. I think that the increased parental age accounts were 11 percent of the increase over this period and that’s a lot and the mechanism by which increased parental ages expressing itself I think likely largely genetic. I think the tricky part is going to recognize that it would be harder now to find that 50 percent. It would look like it should be some toxicological environment that’s shared because of the spacial clustering. Because there’s a very strong process of amplification of the understanding of autism that leads to increased diagnosis as parents learn how to recognize symptoms a very, very, small event that would transform the environment five years ago, ten years ago, even you could imagine, 40 years or 50 years ago, when the moms of children with autism now were in utero as eggs- a very small event could cascade into a larger epidemic now.

Dr. Insel: So what do you tell parents who ask about this if you have friends who have autistic children and they say “What’s going on here? Why this epidemic?” What do you say in response?

Dr. Bearman: Well, I think parents are struggling to just enormously difficult to have a child with autism. It makes it very hard. I think parents are naturally searching for explanations, and I think that the message now is the search for a quick and dirty explanation might not be advancing science.

Dr. Insel: Thank you very much.  Good discussion.