Kevin Drum and Chris Mooney have already posted excellent summaries of this neat study of motivated numeracy.  You should read them.  But if you prefer the digest version: Even unusually numerate people take off their thinking caps when the numbers are ideologically inconvenient.  Here’s another neat graph from the original paper:


On the non-ideological question about the effect of ointment on a rash, ideology has no effect on accuracy, holding objective numeracy fixed.  On the ideological question about the effective of gun control on crime, however, ideology has a big effect – and the effect rises with numeracy.

What does it all mean?  Mooney calls the results “Enlightenment destroying”:

The Scottish Enlightenment philosopher David Hume
famously described reason as a “slave of the passions.” Today’s
political scientists and political psychologists, like Kahan, are now
affirming Hume’s statement with reams of new data.

But this is a gross misinterpretation of what the Enlightenment was all about.  The Enlightenment claim was never, “Everyone is already reasonable.”  Voltaire himself lamented that, “Common sense is not so common.”  The Enlightenment claim, rather, was “Most people aren’t reasonable – and the evils of the world are part, the predictable consequence of their irrationality.”  The evidence on motivated numeracy doesn’t justify fatalism.  It should instead inspire commitment to epistemic Puritanism – an ethic of intellectual self-control, dispassion, and disdain for groupthink.