Someone on Quora asked me this question, and I think it is an important one. Here’s my answer.
Copyright laws are complicated. One would best seek legal advice to insure that a work is or is not in the public domain, rather than rely on partial answers. The US Copyright Office publishes explanations of regulations. Below are partial excerpts from the office, and a link to more information.
Works created on or after January 1, 1978
The law automatically protects a work that is created and fixed in a tangible medium of expression on or after January 1, 1978, from the moment of its creation and gives it a term lasting for the author’s life plus an additional 70 years. For a “joint work prepared by two or more authors who did not work for hire,” the term lasts for 70 years after the last surviving author’s death. For works made for hire and anonymous and pseudonymous works, the duration of copyright is 95 years from first publication or 120 years from creation, whichever is shorter (unless the author’s identity is later revealed in Copyright Office records, in which case the term becomes the author’s life plus 70 years).
Works in Existence but Not Published or Copyrighted on January 1, 1978
The law automatically gives federal copyright protection to works that were created but neither published nor registered before January 1, 1978. The duration of copyright in these works is generally computed the same way as for works created on or after January 1, 1978: life plus 70 years or 95 or 120 years, depending on the nature of authorship. However, all works in this category are guaranteed at least 25 years of statutory protection.
If you are in doubt, I encourage you to seek a lawyer or study the Library of Congress rules.