The counterintuitive finding that more information is associated with worse investment performance is one of the most robust and most practically important results in behavioural finance. It challenges the intuition that better-informed investors should make better decisions, and forces an examination of what kind of information actually improves investment outcomes and what kind generates overconfidence, excessive trading, and the other behavioural pathologies that degrade returns.
The classic demonstration of this phenomenon is the study by Paul Slovic, who showed that horse racing handicappers' confidence in their predictions increased substantially as they were given more information about horses and races, while their actual predictive accuracy remained flat. Each additional piece of information increased their certainty without improving their accuracy—a pattern that the investment research has replicated in financial markets. More information makes investors feel better-informed; it does not make them more accurate, and the increased confidence it generates leads to larger position sizes and more frequent trading, both of which are associated with worse returns.
The specific mechanism involves the difference between information that is genuinely decision-relevant and information that creates the feeling of understanding without providing genuine predictive insight. Most of the additional information that more-informed investors possess falls into the second category: detailed knowledge of a company's products and management style, extensive familiarity with industry dynamics, careful attention to macroeconomic commentary, thorough analysis of historical price patterns. Each of these provides a sense of comprehensive understanding; none consistently translates into superior predictions about future price movements, because the market has already processed the same information and priced it in.
The additional confidence that comes from more information is, in this context, the specific danger. The investor who has done extensive research on a company feels more certain about its prospects than the one who has done less research—even though the extensive research does not provide meaningfully better insight into the stock's future performance. This additional certainty drives larger position sizes that are not justified by the actual information advantage, more active trading in response to news that the less-informed investor would ignore, and greater resistance to contrary evidence because the extensive research has created a strong commitment to the initial thesis.
The type of information that actually improves investment outcomes is qualitatively different from the type that most investors accumulate. Useful information for long-term investing includes: a deep understanding of business economics and competitive dynamics, a rigorous framework for valuation, a thorough knowledge of financial history and market cycles, and a clear-eyed assessment of one's own psychological biases. None of these is obtained from reading more financial news or conducting more detailed short-term research; all of them develop slowly, through study and experience, and provide genuine improvements in the quality of investment judgment without generating the false confidence that additional market information produces.