Here’s the latest paper on the topic, from Michael Waldman, Sean Nicholson, and Nodir Adilov. Using a natural experiment to rule out the possibility of reverse causation, the authors find “a strong negative correlation between average county-level cable subscription rates when a birth cohort is below three and subsequent mental retardation diagnosis rates, but a strong positive correlation between the same cable subscription rates and subsequent autism diagnosis rates.”
In other words, TV watching decreases the rate of mental retardation but increases the rate of autism. The authors hypothesize that, consistent with earlier research, “for the typical child television watching at age two improves cognitive development and decreases diagnoses of mental retardation.”
In contrast, TV might be particularly bad news for kids vulnerable to autism:
[W]e believe the most likely explanation is that there is a genetically vulnerable group for which early childhood television watching serves as a trigger for autism. It has long been known that autism has a strong genetic component (see e.g., Rimland (1964) and Folstein and Rutter (1977)). So one explanation for our autism results is that at least for some children diagnosed with autism it is not the genetics alone which triggers the condition, but rather the genes create a vulnerability to autism that is sometimes triggered by early childhood television watching.