Availability bias prevents us from setting our prior beliefs appropriately, whereas confirmation bias stops us from updating them for new information. - David Franklin

But few of us really stop to ask ourselves what new information might change our minds, especially on questions of politics and identity. We fall into the trap of confirmation bias, in which rather than updating our prior beliefs, we mould new information into a form that will confirm them.

I’d like anyone who was born on March 22nd to stand up.” Two students stand up. They link eyes from across the room, and smile. The room laughs. “Now, anyone born on June 14th.” Another two stand up. The room laughs again. “Now, anyone born on August 2nd.” Two more: the class is laughing at each pair, and re-examining their original takes on how low the probability was likely to be. They are learning through experience about human fallibility with estimating probabilities, and at the same time learning something about their classmates. “How about December 21st?” This time, three students stand up. This gets a big laugh for two reasons: firstly, it is unexpected, and secondly, it helps to ram home the idea that in a class that size (there are about 3,000 pairs in a class of 80), there are likely to be “coincidences” everywhere.

But everyone who guessed in this way went low, rather than high. It suggests that there is some form of systemic bias in play.