Moral luck describes circumstances whereby a moral agent is assigned moral blame or praise for an action or its consequences even if it is clear that said agent did not have full control over either the action or its consequences. This term, introduced by Bernard Williams, has been developed, along with its significance to a coherent moral theory, by Williams and Thomas Nagel in their respective essays on the subject.
Broadly speaking, human beings tend to correlate, at least intuitively, responsibility and voluntary action. Thus, the most blame is assigned to persons for their actions and the consequences they entail when we have good cause to believe that both:
Conversely, there is a tendency to be much more sympathetic to those who satisfy any of the following conditions:
Parenthetically, the above criteria do not correlate exactly with moral praise – while it may be true that one can, and should assign a good deal of moral praise to those who had performed a good action, or an action entailing good consequences, completely on their own volition and uncoerced, it is debatable that the same distinction holds for involuntary actions that happened to turn out well or happened to produce good outcomes.
This correlation between responsibility and voluntary action is acceptable to most people on an intuitive level; indeed, this correlation is echoed in American and European law: for this reason, for example, manslaughter, or killing in self-defense carries a significantly different type of legal punishment (i.e., formalized moral blame) than premeditated murder.
Given the notion of equating moral responsibility with voluntary action, however, moral luck becomes a problem. This problem is illustrated by an example that many moral luck philosophers employ – that of a traffic accident.
There are two people driving cars, Driver A, and Driver B. They are alike in every way. Driver A is driving down a road, and, in a moment of inattention, runs a red light as a child is crossing the street. Driver A slams the brakes, swerves, in short does everything to try to avoid hitting the child – alas, the car hits and kills the child. Driver B, in the meantime, also runs a red light but, since no one is crossing, gets a traffic ticket but nothing more.
If a bystander were asked to morally evaluate Drivers A and B, there is very good reason to expect them to say that Driver A is due more moral blame than Driver B. After all, Driver A's course of action resulted in a death, whereas the course of action taken by Driver B was quite uneventful. However, there are absolutely no differences in the controllable actions performed by Drivers A and B. The only disparity is that in the case of Driver A, an external uncontrollable event occurred, whereas it did not in the case of Driver B. The external uncontrollable event, of course, is the child crossing the street. There is no difference at all in what the two of them could have done – however, one seems clearly more to blame than the other. How does this occur?