If a work is deemed "joint," each author has undivided ownership in the work as intellectual property, and none of the authors can unilaterally sell or license the work for use. In order to be considered "joint," a work must be "prepared by two or more authors with the intention that their contributions be merged into inseparable or interdependent parts of a unitary whole." 17 U.S.C. § 101. Courts have interpreted this to mean that all putative joint authors must intend to make a joint work at the time of the creation of that work. See Childress v. Taylor, 945 F.2d 500, 505-507 (2d Cir. 1991).
In order to be considered a joint work, each author's work must be
(1) independently copyrightable (for example, mere "editing"
creates a "derivative" work, not a "joint" work). See Ashton-Tate v. Ross, 916 F.2d 516 (9th Cir. 1990) but also
(2) inseparable from the whole. (If it is separable, the work is "collective.")
For a thorough overview of the Joint Author doctrine, see Laura G. Lape, A Narrow View of Creative Cooperation: The Current State of Joint Work Doctrine, 61 Alb. L. Rev. 43 (1997).
(1) if it is created "by an employee in the regular scope of employment"
(2) if it is one of 9 types of commissioned works created by an independent
contractor if the parties have agreed in writing that it is a work for
(a) collective work
(b) motion picture or other audiovisual work
(d) supplementary work
(f) instructional text
(h) answer to a test
17 U.S.C. § 101
Courts have not decided whether web sites should be included in section (b) "audiovisual work." The seminal "work for hire" case, CCNV v. Reid, 490 U.S. 730 (1989), explored the meaning of the work-for-hire doctrine's first prong.
Under Reid, the designation of a work created "in the ordinary course of employment" depends on a set of factors, including: the hiring party's right to control the substance of the work; skill required to create it; source of the materials used in creation, location and hours of the labor, the duration of the relationship and right of the hiring party to assign additional work; method of payment; business of the hiring party; and employee benefits and tax treatment. Using that test, the Court in Reid decided that a sculpture commissioned by CCNV was not a "work for hire" because only one of the factors identified clearly leaned in favor of CCNV; but that the sculpture may have been a joint work between CCNV and Reid.
In an interesting exception, Professors who work full-time for universities do not sacrifice copyrights in the fruits of their research, although that research and the writings that result are done in the course of their employment at universities.