This paper formally explores the optimal length of copyright protection when the value of an intellectual work changes over time due to depreciation and value-enhancing ex-post investments. The first main finding is that, in the case of a single project, granting infinitely-lived copyright protection maximizes social welfare when the return on ex-post investments is high relative to the return on the initial investment. We also provide simulation results of our model for the case of multiple heterogeneous projects that show how social welfare varies with the length of copyright protection and the returns on initial and ex-post investments. We then consider what our framework says concerning the social-welfare effects of the 1998 Copyright Term Extension Act. Here we show that, depending on the importance of ex-post investments, the act may have either increased or decreased social welfare. Our final analysis considers the social-welfare implications of replacing fixed-length copyright protection with Landes and Posner's (2003) idea of indefinitely-renewable copyright protection. We find that implementing indefinitely-renewable copyright protection frequently increases social welfare provided the returns on ex-post investments are sufficiently large. We also provide a brief history of Disney's Mickey Mouse and argue that the history of that character matches quite well with the predictions of our theoretical approach.
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