I think there are a few factors at work: 1) In the developed world for people with good incomes and health insurance, people live in relative safety and misjudge risk. Many diseases are rare or nonexistent, so are not viewed as risks as compared to autism (which may receive more media attention, than say the last measles outbreak). 2) Difficulties with distinguishing correlation/causation 3) Interested parties intentionally muddying the waters. The tobacco industry is a prime example of this and Merchants of Doubt makes for excellent, albeit chilling, reading. 4) A desire for control--it's easier to blame someone else if your kid gets autism then to ascribe it to unknown causes. For a parent (who believes that the risk of not vaccinating is small), it may be more satisfying to believe that they can protect their kids against autism, rather than face the fact that either due to unknown environmental or genetic (or an interplay between the two) factors, their child may become (or was diagnosed with) autism and there is nothing that they can currently do (have done) to prevent it....