Hank Green looks into the mindset of people who shun vaccines, trying to get to the bottom of the science behind the anti-vaccination movement:
Once their mind has been made up, confirmation bias sets in. Confirmation bias is simply our tendency to more readily, and with less scrutiny, accept information, anecdotes, and world views that confirm our existing beliefs. And, again, it is a completely normal thing that every person does. Indeed, trying to convince someone that a previously held belief is incorrect has been proven to actually increase their affinity for that idea. And so a community is born, and the safety of vaccines is called into question. And once the procedure for getting a vaccine goes from the doctor telling you that it is now time for a vaccine — and 99% of parents agreeing because that person went through medical school — to it being a question to ponder, vaccination rates will go down.
(via Kottke. Thanks to Seth for the link)