The researchers say that, if these findings were extrapolated to the wider population, for every three known cases of autism spectrum, there may be a further two cases that are undiagnosed.
Professor Baron-Cohen and I had the following exchange about the autism ‘epidemic’:
KL: What would you say to someone who says that your paper is strong evidence of an ‘autism epidemic’ (because you know they will)?
SBC: I think the term ‘epidemic’ of most value in relation to contagious diseases, which autism is not.
KL: Can I rephrase my question? Would you say your findings support the idea that there has been a true rise in prevalence? As oppose to the seven items you say have caused a seeming rise in autism earlier in your paper?
SBC: There has been a real rise in prevalence but what is at issue are the causes of this rise. In the paper we summarize the quite ordinary factors that might have driven the rise, such as better recognition, growth of services, and widening diagnostic criteria.
What appears to be missing in this exchange is any reference by Professor Simon Baron-Cohen to the environmental factors contributing to autism even though he himself has acknowledged the role of environmental factors in the past. In an exchange with a parent published on the One Click Group site Professor Baron-Cohen stated:
The One-Click Group seems to be a website for those who want to see more research into environmental risk factors in autism, and to me this seems to be a very worthy agenda. We know that autism is not 100% genetic in origin, since in the case of identical twins (who share 100% of their genes), there are instances of one twin having autism and the other not having it. In fact, the likelihood of the co-twin also having autism where one of them has it (in monozygotic (MZ) pairs) is about 60%. This means that there must be some non-genetic (i.e., environmental) factors that are part of the cause of autism. ...... I hope the above statement shows clearly and unambiguously that I regard autism as most likely the result of a gene-environment interaction.
Professor Baron-Cohen made the same point in a December 2007 interview piece published on TimesOnLine, Freedom of Expression:
Studies of twins have established that it is not 100 per cent genetic, since even among identical twins, when one has autism, the likelihood of both twins having autism is only about 60 per cent. This means there must also be an environmental component, but what it is remains unknown.
It is not clear whether (1) Baron-Cohen went on to comment in his email exchange with Kevin Leitch about the environmental factors and those comments were not included in the excerpt published at LB/RB or whether (2) Kevin Leitch did not bother to ask about such environmental factors.