Greg Mankiw's Blog: Supply, Demand, and Healthcare Reform

Here are some basic principles of supply and demand: If a government policy increases the demand for a service, the price of that service tends to rise. If the government prevents prices from rising, shortages develop. The quantity provided is then determined by supply and not demand. In the presence of such excess demand, the result could be a two-tier market structure. Consumers who can somehow pay more than the government-mandated price will be able to purchase the service, while those paying the controlled price may be unable to find a willing supplier.

[From Greg Mankiw's Blog: Supply, Demand, and Healthcare Reform]