A few months ago, I offered a completely speculative hypothesis on television and autism:
So how might TV be one of the causes of the "autism epidemic"? A possible answer focuses on the way the newborn brain organizes itself in response to the stimuli it receives. If an infant's world is suffused with cartoons and television shows instead of normal human interaction, then it wouldn't be so outlandish to imagine a brain that is ill-equipped at understanding and interpreting other people. In other words, autistic children are bad at generating a theory of other people's minds because they didn't have very much practice at it. Their mirror neuron systems missed some critical period of experience.
Now, researchers have gone and found some evidence for this idea. After looking at the data, social scientists at The Johnson School at Cornell University noticed a striking correlation between exposure to television at an early age and rates of autism in three separate states. They concluded that their "findings are consistent with early childhood television viewing being an important trigger for autism":
Autism is currently estimated to affect approximately one in every 166 children, yet the cause or causes of the condition are not well understood. One of the current theories concerning the condition is that among a set of children vulnerable to developing the condition because of their underlying genetics, the condition manifests itself when such a child is exposed to a (currently unknown) environmental trigger. In this paper we empirically investigate the hypothesis that early childhood television viewing serves as such a trigger. Using the Bureau of Labor Statistics' American Time Use Survey, we first establish that the amount of television a young child watches is positively related to the amount of precipitation in the child's community. This suggests that, if television is a trigger for autism, then autism should be more prevalent in communities that receive substantial precipitation. We then look at county-level autism data for three states - California, Oregon, and Washington - characterized by high precipitation variability. Employing a variety of tests, we show that in each of the three states (and across all three states when pooled) there is substantial evidence that county autism rates are indeed positively related to county-wide levels of precipitation. In our final set of tests we use California and Pennsylvania data on children born between 1972 and 1989 to show, again consistent with the television as trigger hypothesis, that county autism rates are also positively related to the percentage of households that subscribe to cable television. Our precipitation tests indicate that just under forty percent of autism diagnoses in the three states studied is the result of television watching due to precipitation, while our cable tests indicate that approximately seventeen percent of the growth in autism in California and Pennsylvania during the 1970s and 1980s is due to the growth of cable television. These findings are consistent with early childhood television viewing being an important trigger for autism. We also discuss further tests that can be conducted to explore the hypothesis more directly.
It's very important to note that this data is still extremely tenuous, and that a statistical correlation does not imply causation. Nevertheless, I'm still impressed by the fact that "approximately seventeen percent of the growth in autism in California and Pennsylvania during the 1970s and 1980s is due to the growth of cable television." Crazy, if true.
Hat tip: Lisa Daniel
Update: Here's the Slate article.
You are right to point out that this work is tenuous. The science blog world lately is full of statements that science is by definition tentative. Far from being a weakness, that's actually the strength of science done right.
There is another important caveat to this work, however. It is not peer reviewed. If it were, and it were published in a reconized journal, we could treat it as one large stop LESS tenuous.
The authors' CVs list it as a "working paper" and for its publication status they simply use the quaint "mimeo." Again, there is nothing wrong with this. But if the science press or the blogosphere does not distinguish between self-publication and peer review, an important mechanism assuring public trust in science is lost.
Its less compelling than the "Autism Fries" theory (i.e. it is BS). If this TV hypothesis is true, how do you account for the twin studies (summaries taken from Wikipedia):
* Ritvo-Freeman-Mason-Brothers-Mo-Ritvo (1985)
This study of twins enrolled with the UCLA Registry for Genetic Studies found a concordance of 95.7% for autism in 23 pairs of MZ twins, and 23.5% for 17 DZ twins.
* Steffenburg-Gillberg-Hellgren-Andersson-Gillberg-Jakobsson-Bohman (1989)
In this study, Nordic countries were screened for cases of autism. Eleven pairs of MZ twins and 10 of DZ twins were examined. Concordance of autism was found to be 91% in MZ and 0% in DZ pairs. The concordances for "cognitive disorder" were 91% and 30% respectively. In most of the pairs discordant for autism, the autistic twin had more perinatal stress.
* Bailey-Le-Couteur-Gottesman-Bolton-Simonoff-Yuzda-Rutter (1995) 
The study reexamined a British twin sample and found 60% concordance for autism in MZ twins vs. 0% concordance for DZ. It also found 92% concordance for a broader spectrum in MZ vs. 10% for DZ. The study concluded that "obstetric hazards usually appear to be consequences of genetically influenced abnormal development, rather than independent aetiological factors."
Researchers have noted that this study contained many methodological improvements, a possible reason why it is frequently cited.
* Scourfield-Martin-Lewis-McGuffin (1999) 
This study looked at social cognitive skills in general-population children and adolescents. It found "poorer social cognition in males", and a heritability of 0.68 with higher genetic influence in younger twins.
* Constantino-Todd (2000) 
This study looked at reciprocal social behavior in general-population identical twins. It found a concordance of 73% for MZ, i.e. "highly heritable", and 37% for DZ pairs.
Presumably twins in the same household, whether identical or not, watch the same amount of TV
From the summaries, the studies you site do not appear to be at all inconsistent with an environmental trigger being important to the development of autism; they merely show that genetic factors are very important as well. It is entirely possible (though IMO unlikely) of many of the twins where both developed autism, that neither would have developed autism had they not been exposed to television.
"From the summaries, the studies you site do not appear to be at all inconsistent with an environmental trigger being important to the development of autism; they merely show that genetic factors are very important as well. It is entirely possible (though IMO unlikely) of many of the twins where both developed autism, that neither would have developed autism had they not been exposed to television."
You cannot obviously disprove a negative, but the huge difference in rates between fraternal and identical twins is telling. There is no widely accepted, peer reviewed scientific evidence that establishes an environmental trigger for ASD. Television is as equally (un)likely as french fries, thimerisol, MMR or alien abduction as an environmental trigger.
Not Mercury gives this study the treatment it deserves:
BTW if anyone needs another reason to ignore this study, the authors are from Cornell's business school and have no scientific background
You cannot obviously disprove a negative,
Well no, but it would certainly be possible to put some upper limits on how big an effect television has on autism--as has been done for the proposed mercury-autism link. Whether this hypothesis merits such research is of course another matter.
... but the huge difference in rates between fraternal and identical twins is telling.
I suppose so. I'm reluctant to generalize from environmental differences between members of twin pairs to environmental differences between children in general.
There is no widely accepted, peer reviewed scientific evidence that establishes an environmental trigger for ASD.
I'll take you word for it.
Television is as equally (un)likely as french fries, thimerisol, MMR or alien abduction as an environmental trigger.
Hmmm. The reasons given in the paper for suspecting television didn't impress me.
I'm not qualified to critique the paper. I would be more impressed if it were peer-reviewed and in the relevant scientific journal. (Although I am mindful that a stamp of approval is not the point of the journals.) I am surprised that they included per-country dummy variables, and per-cohort dummy variables, but never both together. I find it easy to believe that the correlation is simply a coincidence from broad patterns of both autism and rainfall in both time and geography.
I ran across another critique of the paper:
Oh, and the Not Mercury url changed: