Other theories address the rise of autism in recent times. They suggest that the rise of visual media and the increasingly central role of visual information processing in the breakdown of language contributes to the increase of autism. Other theories involve the effect of toxins and poisons on neural development. It has been suggested that high levels of heavy metals such as lead may be a causal factor, and lead poisoning has been strongly linked to some cases of severe autism.
One of the most controversial claims, along these lines, is that conditions such as autism and Asperger's are caused by adverse side-effects of immunization—particularly the so-called triple antigen vaccine—and from the heavy-metal preservatives that in the past were used in their manufacture. This theory has a degree of popular currency and has been discussed in a number of documentaries on the subject. It has also been supported by some recent research that indicates that glutathione, a natural antioxidant, may be unusually low in autistic people; this may explain why the tiny amounts of mercury found in vaccines might be detoxified in the bodies of some children, but cause damage that leads to autism in a few. If this theory has merit, a blood test for autism, measuring glutathione levels, might be in the offing in the near future. However, critics argue that this theory fails to address the basic empirical observation that Asperger's and related disorders are overwhelmingly predominant in males, yet children of the two sexes are immunized in roughly equal numbers.
A lesser known theory is that Asperger's and related syndromes may, in part, perpetuate genetic traits which were once highly advantageous to survival. One of the most notable characteristics of many people with Asperger's is that even very young children display a prodigious and often eidetic (photographic) memory, with the ability to precisely recall very large amounts of music, dialogue and speech after even a single hearing.
Although being able to precisely memorize and recall vast amounts of detail, such as cricket scores, dinosaur facts or television-show dialogue, may seem little more than an unusual trait in some societies, it is important to realize that such a skill would have had a high value in pre-literate society. The ability to accurately memorize and re-tell stories, myths, histories and other important oral traditions — such as directions to a remote food or water source — could have meant the difference between life and death before the invention of writing.
