It's not a question about contract. It's about how joint copyright works. I know what the arguments would be, but I'm not sure how the court would rule. The hy

It's not a question about contract. It's about how joint copyright works. I know what the arguments would be, but I'm not sure how the court would rule. The hypothetical depends on legally making a fork, and the GPL (as an example) only gives permission to fork if you agree that any distribution will be under the terms of the GPL. If you violate those terms, you've committed copyright infringement. But can a joint copyright owner be guilty of infringing his own work? I remember Eric Raymond discussing a commercial copyright infringement case where a contractor was hired to update a program, and started distributing the updated program without permission. This was his argument. However, he hadn't made significant changes to the program. Using your words, the program was several orders of magnitude larger than what the contractor added. The court got creative, and declared the contractor only had a collective copyright -- like somebody who has copyright on a single article in an encyclopedia -- and not a joint copyright -- even though his changes were meant to be combined with the original work to create a single work and not a collection of works. A collective copyright owner only has copyright in his contribution. You are right that courts would be reluctant to allow the tail to wag the dog, but there is some threshold where the contributor has made enough changes that he has joint copyright, and at that point he can do what the law says a joint copyright holder can do.