Jonah Lehrer muddles it:
A new paper demonstrates, once again, that the human brain is the ultimate category buster, blurring the lines of good and bad, black and white, until everything is gray. The reason is that our behavior is deeply contextual, profoundly influenced by our surroundings and immediate situations. Whether or not we're able to resist sin, then, might depend more on the details of the sin - and whether or not it triggers our automatic urges - then on the strength of our moral fiber.
That, at least, is the tentative conclusion of a clever new fMRI study by Joshua Greene and Joe Paxton at Harvard University, who argue that sometimes we do the right thing because the wrong thing simply isn't tempting, even if it leaves us better off. Consider a hypothetical wallet, stuffed full of cash, which you find on the subway. Our moral intuitions (influenced by Genesis) tell us that everyone wants to take the money and run, that we're all attracted by the possibility of unearned cash. But this latest study suggests that, at least for the people who take the wallet to the police, there is no temptation to resist. They don't steal because they don't want to steal; telling the truth isn't hard work. They are living, in other words, in a state of moral grace, at least when it comes to the wallet. (Interestingly, Greene and Paxton found that people who behaved dishonestly in the experiment exhibited more activity in brain areas, such as the prefrontal cortex, associated with self-control. In other words, they might be trying harder to resist, but it's doing no good.)