Seeking What We Already Believe
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Confirmation bias is the tendency to search for, interpret, favour, and recall information in a way that confirms or supports one's prior beliefs and hypotheses. It is arguably the most pervasive and well-studied cognitive bias, with a research history stretching back to the 1960s.
The foundational experimental demonstration came from British psychologist Peter Wason in 1960. In the Wason selection task, participants are shown four cards (say: E, K, 4, 7) and told the rule "if a card has a vowel on one side, it has an even number on the other." Asked which cards to turn over to test the rule, most people choose E and 4 — finding confirming evidence. The logically correct answer is E and 7 (to find disconfirming evidence — does 7 have a vowel?). Most people systematically avoid seeking information that could falsify their belief.
Confirmation Bias in Three Stages
Confirmation bias operates at every stage of information processing:
Seeking: We actively look for information that supports what we already think. An investor who believes a company is undervalued will search for bullish analyst reports rather than bearish ones.
Interpreting: Ambiguous information is read in light of prior beliefs. The same quarterly earnings report can be interpreted as "managed through difficulty" by a bull or "masking structural problems" by a bear — both seeing the same numbers.
Remembering: We more readily recall events, facts, and data that are consistent with our existing views. An investor who has held a thesis for years will remember the supporting evidence more vividly than the disconfirming events.
The Role of Motivated Reasoning
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Confirmation bias is amplified when beliefs carry emotional weight. Motivated reasoning — a term developed by psychologist Ziva Kunda in a 1990 paper — describes the process by which people reach conclusions they are emotionally invested in and then construct apparently rational justifications. The conclusion comes first; the argument is built backward.
In investing, motivated reasoning is the mechanism behind the common experience of "doing due diligence" that systematically confirms a decision already made. Genuine due diligence looks for disconfirming evidence; motivated due diligence looks for permission to do what you already wanted to do.
Echo Chambers and Filter Bubbles
Confirmation bias is no longer purely an individual cognitive problem. Digital platforms algorithmically serve content that users engage with — and people engage most with content that confirms their views. The result is the filter bubble (a term coined by internet activist Eli Pariser in 2011): a personalised information environment that reflects and reinforces existing beliefs.
For financial decision-making, the practical consequence is an information diet that systematically underweights disconfirming evidence. An investor who follows only bullish voices on financial social media will be genuinely surprised by negative outcomes that were well-documented in sources they never saw.
Countering Confirmation Bias
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The most direct counter is to actively seek disconfirming evidence — to ask "what would have to be true for me to be wrong?" before committing to a financial decision. Red-teaming a thesis (asking someone to argue against it as forcefully as possible) and consulting bearish analysis before buying an asset both operationalise this principle. The goal is not to become permanently sceptical, but to test beliefs against the strongest available counter-evidence before acting on them.