Overconfidence is one of the most important biases in financial decision making and commonly associated with excessive trading and asset price volatility. So far, most of the finance literature takes overconfidence as a given, “static” personality trait. Using a new experimental design, we show that trader overconfidence is endogenous and co-moves with asset prices; when asset prices go up, overconfidence rises, and when asset prices go down, overconfidence falls. Larger fluctuations in asset prices are met by larger changes in overconfidence. Hence, our results point towards a feedback loop in which overconfidence adds fuel to the flame of existing bubbles.