Situationism, Virtues, and Control
The situationist critique of virtue ethics runs as follows. Recent social psychology has shown that the factors governing human behavior are largely situational rather than characterological. If Robin behaves generously and Sanjay behaves greedily in some particular case, that's more likely to be due to differences in their situation than to differences in their personality. (Maybe Robin has more money, or is in a better mood, or had a different prior interaction with the recipient of her generosity.) For simplicity, we might imagine a radical situationism according to which everyone behaves identically in identical situations, and situation-transcending character or "personal virtue" has no explanatory force at all. We might condemn Osama bin Laden or Paris Hilton (taking examples of different moral gravity!), but any of us, put in the same situation as they (not necessarily from birth, but in some reasonably restricted time-frame), would behave just the same way. Virtue ethics, which stresses the cultivation of personal virtues, would then appear to be based on a psychological impossibility.
Radical situationism is, of course, too extreme. But even if stable cross-situational character plays some role in our behavior, if that role is limited and much more derives from situation than from character, a moral psychology focusing on personal virtue may seem to miss the psychological reality. (John Doris and Gilbert Harman have advanced views of roughly this sort.)
Friends of virtue ethics have tended either to attack situationist psychology -- and, indeed, the empirical issues aren't entirely settled -- or have suggested that even if most people are blown by the winds of situation, the moral ideal can and should be a matter of rising above that sort of weakness of character (as advocated, I'd suggest, by Mengzi in his view of the "unmoved heart" [2A2]).
But I prefer the following response: Suppose even a very radical situationism is true. Suppose, to be concrete, Maria and Mary behave very differently but only due to differences in situation. Maria is a church-going kindergarten teacher who consistently behaves kindly and generously, while Mary is a hard-partying corporate defense attorney who likes to defraud both clients and plaintiffs of all she can; yet, were their situations reversed within weeks each would behave like the other. (Let's set aside the prickly question of whether church-going actually has any positive impact on behavior!)
We can still say this: Maria consistently behaves generously, Mary greedily. But can we truly call Maria "generous" and Mary "greedy" if their behavior is so contingent on situation? Well, supposing the situation is stable, I don't see why not! And moreover, Maria works to keep her situation stable: She chooses to go to church and to be a kindergarten teacher. Maybe she even knows that were she to start hanging around a different crowd or pursue a different career that would poison her character and for that very reason avoids it!
Because she exerts such control, we can hold Maria responsible and praiseworthy for her generosity (and Mary blameworthy for her greed). And the point generalizes. We can acknowledge the situational, even the radical situational, contingency of our character traits without abandoning the moral value of thinking terms of such traits and aiming to achieve such traits ourselves. We need only have enough control over our situation to ground stability and responsibility.
(I take this view to be in the spirit of Maria Merritt, though I'm not sure the point about control comes out quite as sharply as I would like in the written work of hers I've seen.)