Behavioral biases extend beyond individual decisions to shape entire markets. Herd behavior describes the tendency of individuals to mimic the actions of a larger group, often disregarding their own information or analysis in favor of following the crowd. It can be driven by informational cascades — situations where people sequentially observe others' decisions and rationally choose to follow them, ignoring their own private signals — as well as by social pressure and the career risk of deviating from consensus. Herd behavior helps drive speculative bubbles, where asset prices rise far above fundamental value because each successive buyer chases rising prices based on social proof rather than intrinsic worth. The Dutch Tulip Mania of 1637 and the late-1990s Dot-Com Bubble are historical examples where exuberant buying produced extraordinary prices before dramatic collapses. In the 2008 housing crisis, overconfidence, herd behavior, and mental accounting led homebuyers, lenders, and investors to underestimate risk and fuel unsustainable price appreciation.
Overconfidence is among the most pervasive market biases: investors consistently overestimate their own knowledge, abilities, and the precision of their predictions. Overconfident investors trade excessively, believing they can outperform the market, and typically earn lower net returns after transaction costs. Closely related is myopic loss aversion, the combination of loss aversion with frequent portfolio evaluation. Investors who check returns often feel the pain of short-term losses more acutely and consequently take less risk. Benartzi and Thaler used myopic loss aversion to explain the equity premium puzzle — the observation that stock returns have historically exceeded bond returns by a margin too large to be explained by standard risk-aversion models. If investors evaluate portfolios annually and are loss-averse, they will demand a high premium for stocks, matching observed premiums despite rational models predicting lower ones.
Other biases distort investor behavior in distinctive ways. Ambiguity aversion, demonstrated by the Ellsberg Paradox, leads people to prefer known risks over unknown ones, even when expected values are identical; this drives home bias, the tendency to overweight domestic securities despite potential gains from international diversification. In auctions, the winner's curse causes the winning bidder to overpay because the winner is typically the person who most overestimated the item's value. Money illusion leads people to think of money in nominal rather than real terms, distorting spending and wage negotiations when inflation is non-trivial. Fairness preferences also shape market outcomes: in ultimatum-game experiments, responders frequently reject low offers — sometimes below 20% of the total — even though accepting would be financially rational, demonstrating that people are willing to punish unfair behavior at personal cost.