Fairness and fair shares

Some moral principles require agents to do more than their fair share of a common task, if others won't do their fair share - each agent's fair share being what she would be required to do if all contributed as they should. This seems to provide a strong basis for objecting to such principles. For it seems unfair to require agents who have already done their fair share to do more, just because other agents won't do their fair share. The philosopher who has written most about this issue, however, Liam Murphy, argues that it is not unfair to do so, at least in the standard sense of that term. In this article, I give Murphy's reasons for saying this, explain why I think he's wrong, and then say a little about why this issue might be important.