When large denomination bills are preferred in illegal activities, what is the optimal policy response? We construct a dual currency model where illegal activity can be reduced by modifying the payment environment. In our model, legal (goods) traders are indifferent between small and large bills, but illegal (goods) traders face a lower transaction cost of using large bills in comparison to small bills because it is easier to conceal. We show that eliminating large bills can reduce illegal trade and its associated social cost. However, this pooling equilibrium is sub-optimal because the government can collect more seigniorage by allowing illegal traders to use large bills with a lower rate of return. When the transaction cost of using small bills for illegal traders is sufficiently large, a separating equilibrium, where legal traders use small bills and illegal traders use large bills, can maximize welfare by making an implicit transfer from the illegal traders to the legal traders.

当在非法活动中优先使用大面额的纸币时，最佳的对策是什么？我们构建了一种双币种模型，可以通过修改付款环境来减少非法活动。在我们的模型中，合法（商品）交易者对大钞和小票都无动于衷，但是非法（商品）交易者使用大钞比小票面对的交易成本较低，因为它更容易隐藏。我们表明，消除大笔账单可以减少非法贸易及其相关的社会成本。但是，这种集合平衡不是最优的，因为政府可以通过允许非法交易者以较低的收益率使用大额票据来收集更多的铸币税。当为非法交易者使用小额票据的交易成本足够高时，就会产生分离的均衡，