Copyright is a property right, but unlike other forms of property, it cannot be passed down for hundreds of years, because copyrights automatically expire 70 years after the death of the author.

Ownership of ideas and their expression is more complicated than the ownership of tangible property. Central to the ownership of property is the right to exclude all others from its use. However, the spread of ideas is beneficial to all.

The general rule of law is, that the noblest of human productions-knowledge, truths ascertained, conceptions, and ideas- became, after voluntary communication to others, free as the air to common use.

-U.S. Supreme Court Justice Louis Brandeis, dissenting,
International News Service v. Associated Press, 1918

From the beginning, copyright laws have sought to balance authors’ rights with the benefit to the public of the free exchange of ideas and cultural expression, and this has always been done by limiting the duration of copyright. The first copyright act, the Statute of Anne, passed in England in 1710, limited copyrights to 21 years for existing works, and to a 14-year term for new works, with the term renewable for another 14 years. This statute became the model for U.S. laws. Limitations on the duration of copyright are written into the U.S. Constitution.

The Congress shall have the Power … To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries…

-U.S. Constitution, Article I, Section 8

The first Copyright Act of 1790 mimicked the Statute of Anne in providing for a 14-year term of copyright, which could be renewed once. In 1909, this was extended to a 28-year term, which could be renewed once. In 1976, renewal terms were eliminated, and the duration of copyright became the life of the author plus 50 years. This brought the U.S. more in line with the international standards of copyright law in the Berne Convention. In 1998, the U.S. Congress passed the Sonny Bono Copyright Term Extension Act, which increased the duration of copyright to the life of the author plus 70 years.