Some people have suggested (usually to avoid the conclusion of intelligent design arguments) that our universe is just one universe among many, and in fact there's a universe for every possible way things could have gone. Whole TV shows have been based on this claim. Tyler Cowen at Marginal Revolution discusses the ethical implications of the many-worlds thesis [hat tip: Philosophers' Carnival XXVIII] .
He argues that ethical questions would be irrelevant if this view is correct. No matter what you do, someone else just like you is doing each alternative possibility among the choices that were available to you. So if you can do the good thing or the bad thing, it doesn't matter which you pick, because your picking the bad one ensures that the good one will be done, and your picking the good one ensures that the bad one will be done. Either way the resulting multiverse is no different. Your action is simply irrelevant to what the multiverse will be like after your done. So ethics would be irrelevant. I disagree. This view doesn't have that consequence, and Tyler is just assuming something that I wouldn't grant.
The argument is that the net result wouldn't be any different if I did an action usually considered better worse, and therefore the action isn't really better or worse because the consequence of either action would be the same. If consequentialism is true, then the only morally relevant features of the action are its consequences, but anyone who denies consequentialism isn't going to buy this. Just because two actions lead to the same consequences doesn't mean they're morally equivalent. Tyler's argument rests on the consequence being the same, as if that's the only morally relevant issue. Non-consequentialists won't accept that.
But the other problem is one I think a consequentialist might even agree with. Consequentialists don't always believe that my action's moral value depends just on whatever future state of the world occurs afterward. It depends, for many consequentialists, on which aspects of the future state of the world have something to do with my action. If my action causes the bad, then I am to blame. If it causes the good, I'm to be evaluated positively. So if I'm the one who does the bad thing, and some duplicate of me in another part of the multiverse does the good thing, then I'm to blame and he is to be congratulated. So my not doing an action might logically (but not causally) entail someone else doing it, and it would lead to a result that's exactly similar to what happens if I do the other action. But that doesn't mean my action isn't bad if it's the bad one. In the world where I do the bad action, my action is bad. In the world where my duplicate does the bad action, his action is bad. The total amount of good or bad in the universe is irrelevant to whether my action right here is bad.
That means we should blame the one who does the bad thing, even if any choice leads to the same resulting future when you factor in the whole multiverse. This means that, even if this view that we have no evidence for is true, moral evaluation still makes sense.