starkly at 18 - 24 months, sometimes there are signs much earlier (e.g., posture, social interest).  But you can see how a parent would find that correlation suspicious, if they had their child vaccinated on schedule.

is that 'cause' is a close cousin of 'blame'.  People desperately want to uncover any causes of autism we can control in the hopes of keeping our children safer.  Yet no one can bear the thought that their child's autism is in any way their fault.

But what happens if we actually do discover that autism is caused by tylenol, or McDonald's french fries, or TV?  Or anything else that a child receives from a loving parent?  Parents will agonize over every fry they let their baby gnaw on, every episode of Sesame Street they turned on, every low fever they didn't leave alone.  Some parents would accept that they couldn't have known, but for others the knowledge that they were the one to expose their child would be devastating.

And so some theories are more strongly resisted than others, because there is a strong need to believe that it must be something like vaccines, viruses, air pollution, or groundwater contamination.  A cause we can fight to improve without taking blame ourselves for the exposure.