When two or more authors cooperate to create a single work, they might be considered the joint owners of a copyright on the work if they showed an intent to write jointly. The copyright on the work survives the death of any of its authors and generally lasts for 70 years after the death of the last surviving author. After the copyright expires, however, the work will enter the public domain.
Joint Ownership and Multiple Contributors
The copyright to a work may be held jointly by co-authors when they intermingle their contributions and demonstrate an intent to create a joint work. Because a single work was created with contributions from more than one author, however, that doesn't necessarily mean that the work was co-authored for copyright purposes. A book of short stories authored by different people, for example, is not necessarily a joint work. If the work is not a joint work, each author owns a copyright on his individual contribution.
Rules for copyright duration have changed over the years. However, as long as the work was created on or after January 1, 1978, the copyright term for a work created by an individual is 70 years after the death of the author. The date of death of the author determines when the copyright expires, no matter who owns the copyright when the author dies. If the work is considered a joint work, the copyright endures for 70 years after the death of the last surviving author. If the work is not a joint work, the copyright on each individual's contribution expires 70 years after the death of the author responsible for that particular contribution.
Work for Hire Exception
If one or more authors agree in writing to create a work within the scope of an employment relationship, the employer is considered the author of the work, and the duration of the copyright depends on the identity of the employer. If the author is a corporation, for example, the copyright endures for 120 years after the date of creation or 95 years after the date of first publication, whichever is earlier.
When a copyright expires, the copyrighted work enters the public domain. This means that anyone can use the work free of charge and without obtaining permission from the author. Many classic novels, for example, are now in the public domain. Failure to attribute the work to the true author is considered plagiarism, but not copyright infringement. Keep in mind, however, that later adaptations to the work might still be under copyright even after the original work enters the public domain. Also, a work in the public domain in the U.S. might still be protected overseas, which could expose you to an overseas infringement lawsuit if you publish it overseas.