Hart e Zingales. Trecho:
Here, we can learn from a common market practice: margin accounts. In a margin account, an investor buys stock and puts down only part of the cost. When the stock price drops, the broker who extended a loan for the rest of the stock price asks the investor to post new collateral. The investor then has a choice: He can post the collateral, thereby re-establishing the safety of his position, or he can liquidate his holding, allowing the broker to be paid in full.