A frequently cited finding in psychology research illustrates overconfidence with a genuinely simple statistical impossibility: surveys have repeatedly found a large majority of people rate their own driving ability as "above average" — something that, mathematically, can't be true for a genuine majority of any group at once. Overconfidence bias is exactly this: a systematic tendency to overestimate one's own knowledge, skill, or ability to predict outcomes, and it shows up in investing with real, measurable, expensive consequences.
Finance academics Brad Barber and Terrance Odean's research on individual investor behavior found a clear, documented pattern: more overconfident investors trade more frequently, believing their own analysis and timing give them an edge — and that additional trading activity was associated with worse, not better, net investment returns. The mechanism is straightforward: every trade carries real costs (commissions, the bid-ask spread, and often taxes on short-term gains), and more trades simply create more opportunities to make a timing mistake. Confidence in one's own stock-picking or timing ability doesn't, on its own, produce the skill needed to actually overcome those accumulating costs.
Correct moves you up, wrong moves you down — reach 100 to master this lesson.