Good question,Deadlock.
We can argue that the artifacts belong to Peru,as they are part of that area's cultural history.On the other hand,do they not represent some part of Yale's history as well?Another example would be the Rosetta Stone.Sure,it's an Egyptian artifact,but it was non-Egyptian scholars who figured out why the thing was so important.We can recognize its value to Egypt's history,but we can also recognize its value to England and France's academic history.
Still,I would say that the artifacts should be returned to Peru,as Yale really has no need for them at this point.We probably know about as much as we are ever going to learn from the items,so what point is there for Yale to keep them any longer?