After you buy a stock, something quiet and dangerous happens to your reading habits. Articles that argue your position is correct feel insightful and well-researched. Articles arguing the bear case feel sloppy, biased, or written by people who don't understand the business.

The same brain that ten minutes ago was open to either side now operates as a filter — and the filter is invisible to its operator. Charlie Munger called the underlying mechanism 'the most powerful tendency in human psychology' for a reason: it's simultaneously the most consequential cognitive bias in investing and the hardest one to detect in yourself. Confirmation bias is the universal human tendency to seek and weight evidence that supports an existing opinion while filtering out evidence that contradicts it.

The bias is not a deliberate choice; it operates below conscious awareness. The seminal experiments — Peter Wason's 1960 'rule discovery' study and the Stanford death-penalty studies of the 1970s — showed that even highly educated subjects, when asked to test a hypothesis, systematically sought confirming evidence and avoided disconfirming tests.