I’ll be done with grad school in about two weeks and able to start posting more frequently – hooray!  Today’s entry is a response I wrote for a friend who asked why the thimerosol-in-vaccine-causes-autism movement doesn’t go after thimerosol in household products to anywhere the same degree as in vaccines (especially in vaccines that no longer contain thimerosol).  This entry contains speculation, although it’s speculation based in existing cognitive science, primarily in cognitive anthropology work on what cognitive faculties make some beliefs catchier than other beliefs.  In the field this work gets called “epidemiology of beliefs”: what characteristics of human minds some beliefs compelling in a way that others are not?

The anti-vaccine movement comes in part from anecdotes of kids regressing at about the time they get the MMR vaccine. That’s actually accurate; when I was taking a cognitive development graduate class, the percentage estimated to regress in that way was about 25%. There was some speculation at the time that it’s due to kids’ brains undergoing a major reorganization at (coincidentally) about the same time they get the MMR.

(Kids make a bunch of neuronal connections, then prune out the less useful ones, and the speculation was that the children who lost language and social skills had not pruned as extensively. There was some research on head circumference at the time looking at whether kids with autism had greater head circumference – kind of a crude measure if you ask me but it did seem to be panning out.)

My own impression is that the anecdotes about regression after vaccine (but not anecdotes about regression before vaccine) kick off contamination fears in some people. In the literal “we have a mental faculty that’s highly alert to dangerous contamination by non-visible substances” sense. Which then kicks off a search for an essentialized underlying substance that will explain/justify their intuitions (also an extremely common thing for people to do).

What I’m getting at is that thimerosol isn’t a trigger for concerns, thimerosol-in-vaccines is an explanation for them. It’s the endpoint of a search. Why doesn’t it generalize from there? My speculation is that vaccines are required by authority and contact lens solution etc is not, and stories about having dangerous things forced on you are much more mentally catchy and conducive to righteous indignation and fear than are stories about stuff you can voluntarily avoid. So a lot of people don’t know about it, and it’s not that their lack of concern comes from lack of knowledge, but that their lack of knowledge comes from fears of thimerosol-in-household-products never taking root strongly enough to become widespread.