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In some ways similar to selection bias, confirmation bias is the tendency for people to favour information that agrees with their preconceptions or beliefs, regardless of whether it is actually true.

Confirmation bias is an example of a cognitive bias, a human trait, whereas selection bias is typically a methodological error in a scientific study. The results can be similar: evidence against some hypothesis can be suppressed while evidence for it is overemphasized. However, with selection bias the emphasis depends on the selection error that is made, while confirmation bias the emphasis lies with whatever the person’s existing beliefs are, effectively in favour of they would like to be true.

Confirmation bias happens to every human. We all do it. It affects how we perceive things in the present, and it affects how we remember things from the past: we remember the hits and forget the misses.

The Lunar Effect

Consider for example the so-called lunar effect. It is the theory that there are correlations between the lunar cycle and deviant behaviour in human beings. This idea is so old, so part of the culture, that numerous languages have connections between their words for lunatic and lunar. The theory also manifests in legends about werewolves.

Consider the belief that there is an increase in crime rates during full moon. A fifteen month study in Jacksonville, Florida revealed no lunar effect on crime. Where then, do beliefs like these come from? They could be inherited from culture, and then confirmed via confirmation bias, via a process not too different to this:

The Tides of Crime

Countering this bias

In science, confirmation bias is mitigated by explicitly searching for falsifying evidence (trying to prove theories wrong to see if they stand up to scrutiny), and also via the peer review process. Your peers, with different perspectives and biases, can often spot the things you missed due to your own biases. Eloquently expressed by miller back in 2008:

See, science’s solution to confirmation bias isn’t to make oneself into an objective, emotionless observer. Science’s solution is the peer review process. A bunch of humans with knowledge of the facts, they come together with all their different emotions, opinions, and biases, and they produce truth. That is the best solution to confirmation bias, or about as good as we’ll ever come up with.

In our personal lives, however, it is up to us to deal with our own biases. In The Human Animal Post, cpbotha suggests we need to “read the manual” for our bodies (and brains), by which he means the body of scientific knowledge of how humans function. One of the mentioned benefits of this would be understanding our cognitive biases better. If we care enough about truth and reality, then by increasing our awareness of our natural biases, we will be able to avoid being misled by them as often. Of course we won’t always succeed, but it will give us many more opportunities to revise and improve our misguided views.

Wikipedia has a very thorough Confirmation Bias article, including more examples in the Consequences section. For example, on cognitive bias in the financial sector (emphasis mine):

Confirmation bias can lead investors to be overconfident, ignoring evidence that their strategies will lose money.[4][78] In studies of political stock markets, investors made more profit when they resisted bias. For example, participants who interpreted a candidate’s debate performance in a neutral rather than partisan way were more likely to profit.[79] To combat the effect of confirmation bias, investors can try to adopt a contrary viewpoint “for the sake of argument”.[80] One such technique involves imagining that their investments have collapsed and asking why this might happen.[4]

Awareness and acceptance. That’s the first step.