Modigliani and Miller show that the total market value of a firm is unaffected by a repackaging of asset return streams to equity and debt if pricing is arbitrage‐free. We investigate this invariance theorem in experimental asset markets, finding value‐invariance for assets of identical risks when returns are perfectly correlated. However, exploiting price discrepancies has risk when returns have the same expected value but are uncorrelated, in which case the law of one price is violated. Discrepancies shrink in consecutive markets, but persist even with experienced traders. In markets where overall trader acuity is high, assets trade closer to parity.

Modigliani and Miller showed the market value of the company is independent of its capital structure, and suggested that dividend policy makes no difference to this law of one price. We experimentally test the Modigliani-Miller theorem in a complete market with two simultaneously traded assets, employing two experimental treatment variations. The first variation involves the dividend stream. According to this variation the dividend payment order is either identical or independent. The second variation involves the market participation, or not, of an algorithmic arbitrageur. We find that Modigliani-Miller’s law of one price can be supported on average with or without an arbitrageur when dividends are identical. The law of one price breaks down when dividend payment order is independent unless there is arbitrageur participation.

We investigate the relationship between traders’ expectations and market outcomes with experimental asset market data. The data show that those who have high price expectations buy more frequently and submit higher bids, and those who hold low price expectations sell more frequently and submit lower bids. Traders who have more accurate expectations achieve greater earnings. Simulations using only belief data reproduce the pricing patterns observed in the market well, indicating that the heterogeneity of expectations is a key to explaining market activity.