Sunday, July 10, 2011
Autism in Twins : The World Isn't Flat
In a nutshell, this paper found that the number of identical twins who both have autism is lower and the number of fraternal twins who both have autism is significantly higher than once thought. When you consider the findings separately or together, it all points to the fact that environmental factors play a far larger role in autism than was previously thought.
I think the most surprising thing about this study for me is how many people seem to be taken off guard by this finding. This isn't the first recent twin study to show at least some of these results. In fact, I wrote about one that suggested that autism was far more common in fraternal twins than was thought almost two years ago.
But even ignoring recent twin studies, the purely genetic model has, even after decades of searching, failed to find any mutations that could account for more than an minority of cases of autism. As each passing study not only failed to find any single mutation that could account for more than a small handful of case but also (mostly) failed to confirm the small mutations from the last study, it became clear (at least to me) that genetics alone could not determine whether someone developed autism.
Sure, you can take all of the prior studies together, put on your optimistic glasses, and say that you might be able to point to the genetic "cause" for up to 30% of all cases2 of autism. But that presumes that each of the individual "causes" that have been identified are in fact causes and not simply random chance. It also assumes that a given person would only have one of these "causes". Since most of these "causes" have been identified in only an extremely small group of people, I think it is far more likely that most of these mutations are simply chance and have nothing to do causing autism.
But, for the sake of argument, lets say that you could take a random group of 100 people with autism and find the genetic "cause" in 30 of them. That would still leave the overwhelming majority of cases without a known cause. What does it say when about the correctness of a theory when it fails to account for the majority of cases?
In most of science, if you had a hypothesis that has failed to be confirmed by hundreds of studies over several decades, that would be called a failed hypothesis. But when it comes to autism, that is called the prevailing theory. And the reason that this theory has survived is the results of past twin studies that found that autism seemed to be mostly genetic.
Enter the current study that very meticulously shows that identical twins often don't share their autism and that fraternal twins do so more than other siblings. These findings have effectively taken out the foundation that the genetic only theory of autism was based on. So when you consider that not only has the genetic theory failed to find any major results but also that it might rest on a flawed foundation, I think it is very clear that the theory is dead in the water.
Now, before anyone takes this the wrong way, it is still obvious that there are genetic causes of autism and that genetics does still play a large role in developing autism. However, we can now say (with an almost certainty) that environmental factors also play a large role.
The world of autism is no longer flat.
1. Hallmayer, Joachim et al. 2011. “Genetic Heritability and Shared Environmental Factors Among Twin Pairs With Autism.” Archives of general psychiatry 1-8. http://www.ncbi.nlm.nih.gov/pubmed/21727249.
DOI : 10.1001/archgenpsychiatry.2011.76
2. Schaaf, Christian P, and Huda Y Zoghbi. 2011. “Solving the autism puzzle a few pieces at a time.” Neuron 70:806-8. http://www.ncbi.nlm.nih.gov/pubmed/21658575
DOI : 10.1016/j.neuron.2011.05.025