What balance of incentives was Congress attempting to get right when it chose 20 years as the length of time that a patent would last? How have drug companies managed to extend the monopoly on selling a particular drug past the 20-year duration of the patent on that drug? If drug companies can maintain highly lucrative monopolies on existing drugs indefinitely, do they have much of an incentive to invest in R&D to invent new drugs that might not even sell well?