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Growing evidence shows that biological factors affect individual financial decisions that could be reflected in financial markets. Testosterone, a chemical messenger especially influential in male physiology, has been shown to affect economic decision making, and is taken as a performance enhancer among some financial professionals. This is the first experimental study to test how testosterone causally affects trading and prices. We exogenously elevated testosterone in male traders, and tested testosterone’s effect both on their trading behavior in experimental asset markets and on the size and duration of asset price bubbles. Using both aggregated and individual trading data, we find that testosterone administration generated larger and longer-lasting bubbles by causing high bids and the slow incorporation of the asset’s fundamental value.