The number sequence experiment that discovered the bias
In 1960, British psychologist Peter Wason designed an experiment now known as the '2-4-6 task.' He showed his subjects the sequence 2, 4, 6, and told them the sequence followed a specific rule. Their task was to figure out the rule by proposing their own three-number sequences: Wason would tell them, for each proposal, whether it fit the hidden rule or not.
The vast majority of subjects quickly formed a hypothesis (usually 'consecutive even numbers increasing by two') and from then on only proposed sequences that confirmed that hypothesis (8, 10, 12; 20, 22, 24), never proposing sequences designed to disprove it. Wason's actual rule was much simpler and broader than almost anyone managed to test: simply, three numbers in ascending order. Very few subjects discovered it, because almost no one actively tried to prove their own hypothesis wrong.
Why buying a stock turns you into its biggest defender
In finance, confirmation bias shows up with particular force right after making an investment decision: buying a stock triggers an instinct to defend that decision, not to question it. From that moment on, positive news about the company gets actively sought out and easily remembered, while warning signs get minimized, reinterpreted favorably, or outright ignored.
This pattern is especially dangerous because it works exactly backwards from what would help: the more money at stake in a decision, the harder we should work to actively seek out information that contradicts it, and in practice the opposite happens.
A current example: echo chambers in financial forums and social media
investment forums and social media make it easier than ever to surround yourself exclusively with voices that share your exact thesis about a stock, a sector, or a cryptocurrency. Every comment that confirms your position reinforces your confidence; every dissenting voice can be muted, ignored, or dismissed as 'someone who doesn't get the opportunity.'
This mechanism amplified much of the collective enthusiasm we described in our chapter on herd behavior: during the 2021 GameStop frenzy, the r/WallStreetBets forum acted as an almost perfect echo chamber, where doubts about the rally's sustainability got far less visibility than the messages celebrating it.
Why this bias costs more the longer it takes to correct
The real cost of confirmation bias isn't just holding a wrong idea, but holding it for longer than you would have if you'd actively sought out contrary evidence from the start. The longer someone spends defending an investment thesis while ignoring accumulated warning signs, the more money they typically lose when reality finally catches up.
This pattern repeats across several of the major frauds we cover in our major financial frauds series: many of Bernie Madoff's investors actively dismissed warning signs (suspiciously stable returns, lack of transparency) precisely because they had already decided to trust him, and looked for reasons to keep doing so, not to question it.
How to protect yourself from your own confirmation bias
The most effective technique, and the most uncomfortable to apply in practice, is to actively write down the best possible reason to sell each investment you hold, not just the reasons to keep it. If you can't articulate a serious argument against your own position, that's a sign you haven't really analyzed it: you've only confirmed it.
Deliberately following analysts or sources that disagree with your investment thesis, rather than only those who agree with you, is another practical defense, even though it feels psychologically less pleasant than surrounding yourself with like-minded opinions.
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Frequently Asked Questions
It's the tendency to search for, interpret, and remember information in a way that confirms our prior beliefs, while ignoring or dismissing information that contradicts them, formally described by psychologist Peter Wason in 1960.
That, when trying to discover a hidden rule, most people only propose examples that confirm their first hypothesis, instead of actively trying to disprove it, which stops them from discovering simpler or different rules than the one they initially assumed.
Because buying triggers an instinct to reinforce that decision rather than question it: from that point on, positive news gets sought out and remembered more easily, while warning signs get minimized or ignored.
Forums and social media make it easy to surround yourself with voices that already think like you, reinforcing your confidence with every like-minded comment while muting or dismissing dissenting voices, amplifying confirmation bias at a collective scale.
By actively writing down the best possible argument to sell each investment you hold, and by deliberately following sources that disagree with your thesis, not just those that confirm it.