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Why is evidence so hard for politicians?

Like most of us, politicians cherry pick data that agrees with our ideas and disregard evidence that might challenge those ideas.

Ben Goldacre, author of Bad Science: Quacks, Hacks, and Big Pharma Flacks, explains:

So the Minister has cherry picked only the good findings, from only one report, while ignoring the peer-reviewed literature. Most crucially, he cherry-picks findings he likes whilst explicitly claiming that he is fairly citing the totality of the evidence from a thorough analysis. I can produce good evidence that I have a magical two-headed coin, if I simply disregard all the throws where it comes out tails.

Here is what politicians apparently cannot understand: it’s fine to make policy based on ideology, whim, faith, principles, and all the other things we’re used to. It’s also fine for evidence to be mixed. And it’s absolutely fine if your reforms aren’t supported by existing evidence: you just shouldn’t claim that they are.