Fairness seems to require that, in collaborative endeavours, each contributor pitches in with an equal contribution. But in what sense? Consider the old problem of household chores. We're told to split them "50/50", but what exactly are we measuring here?
Simply doing half of the listed chores is no guarantee of fairness, since some jobs may require more work than others. Similarly for spending equal time: a long pleasant job might reasonably be preferred over a short and nasty one. And what if the two people differ in how long it takes them to do a job, which value 'counts'? (Note also the moral hazard: if one spends all day mowing the lawn at a snail's pace, does that get them off the dishes?)
Perhaps what we seek is the nebulous measure of "effort". But, as in schooling, this incentivizes incompetence. (If you're going to be rewarded for your hapless "efforts" regardless, why bother taking steps to improve the efficacy of your future efforts?)
So, it seems like there's not actually any objective fact of the matter, in such a case, as to what a "fair contribution" would be. At least, if a problem can't be quantified, then there's no such thing as "50%" of it. Absent any such ideal result, the dispute must be resolved by bargaining, not inquiry. So, we may think, anything goes so long as both parties are happy with it. But what about lowered expectations or 'adaptive preferences' as a result of exploitation? Wouldn't that still be problematic? Arguably, justice requires that both parties begin with roughly equal bargaining power. (Having said that, I think it's worth emphasizing that - as a general rule - the injustice resides in the background conditions, and not in the mutually beneficial "exploitative bargain" per se.) Perhaps more importantly, in this context, decency requires giving due weight to one's partner's interests too.
In other words, perhaps some contributions are problematic, not due to the resulting distribution per se, but because they indicate selfishness, "free riding", or lack of concern for one's co-contributors. It's not as though there's some quantity (whether fixed or variable) that each person ought to contribute. It's more a matter of character, and dedication to the common project. So perhaps virtue theory would offer a better ethical analysis of this issue. Any suggestions?