Confirmation Bias As Misfire Of Normal Bayesian Reasoning

by Scott AlexanderSSC1 min read13th Feb 20209 comments

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Confirmation BiasHeuristics & BiasesRationality
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From the subreddit: Humans Are Hardwired To Dismiss Facts That Don’t Fit Their Worldview. Once you get through the preliminary Trump supporter and anti-vaxxer denunciations, it turns out to be an attempt at an evo psych explanation of confirmation bias:

Our ancestors evolved in small groups, where cooperation and persuasion had at least as much to do with reproductive success as holding accurate factual beliefs about the world. Assimilation into one’s tribe required assimilation into the group’s ideological belief system. An instinctive bias in favor of one’s in-group” and its worldview is deeply ingrained in human psychology.

I think the article as a whole makes good points, but I’m increasingly uncertain that confirmation bias can be separated from normal reasoning.

Suppose that one of my friends says she saw a coyote walk by her house in Berkeley. I know there are coyotes in the hills outside Berkeley, so I am not too surprised; I believe her.

Now suppose that same friend says she saw a polar bear walk by her house. I assume she is mistaken, lying, or hallucinating.

Is this confirmation bias? It sure sounds like it. When someone says something that confirms my preexisting beliefs (eg ‘coyotes live in this area, but not polar bears’), I believe it. If that same person provides the same evidence for something that challenges my preexisting beliefs, I reject it. What am I doing differently from an anti-vaxxer who rejects any information that challenges her preexisting beliefs (eg that vaccines cause autism)?

When new evidence challenges our established priors (eg a friend reports a polar bear, but I have a strong prior that there are no polar bears around), we ought to heavily discount the evidence and slightly shift our prior. So I should end up believing that my friend is probably wrong, but I should also be slightly less confident in my assertion that there are no polar bears loose in Berkeley today. This seems sufficient to explain confirmation bias, ie a tendency to stick to what we already believe and reject evidence against it.

The anti-vaxxer is still doing something wrong; she somehow managed to get a very strong prior on a false statement, and isn’t weighing the new evidence heavily enough. But I think it’s important to note that she’s attempting to carry out normal reasoning, and failing, rather than carrying out some special kind of reasoning called “confirmation bias”.

There are some important refinements to make to this model – maybe there’s a special “emotional reasoning” that locks down priors more tightly, and maybe people naturally overweight priors because that was adaptive in the ancestral environment. Maybe after you add these refinements, you end up at exactly the traditional model of confirmation bias (and the one the Fast Company article is using) and my objection becomes kind of pointless.

But not completely pointless. I still think it’s helpful to approach confirmation bias by thinking of it as a normal form of reasoning, and then asking under what conditions it fails.

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