William Galston (The Practice of Liberal Pluralism, p. 97) suggests that moral obligation depends at least in part on moral temperament:
The fact that altruism is possible for some individuals does not prove that it would be possible of all, even under the most favourable formative circumstances. On the contrary: I do not have a knock-down argument, but I defy anyone to read Monroe’s depiction of the rescuers [of Jews during the holocaust] without seeing just how exceptional they are, and how misguided it would be to expect such behavior of everyone. The diversity of human character-types constitutes an obstacle to any understanding of altruism as a universal moral obligation. The dictum that “ought implies can” must be applied in a manner that is sensitive to the deep differences of individual moral capacity. For example, the solitary, self-willed courage most rescuers displayed would simply be beyond the powers of most individuals. I would hazard the conjecture that that this crucial difference is not (entirely) the product of upbringing or social context but represents the expression of deepseated differences of temperamental endowment. If true, this conjecture would not reduce our admiration for the rescuers, but it would relax the rigorous judgment we might otherwise want to pass on those who had the opportunity to rescue, but out of fear failed to seize it.
I think this rests on a thoroughly confused understanding of moral blameworthiness. Without defending the idea that all Germans were morally required to be rescuers, I would suggest that no moral duty should depend on a particular individual’s moral capacity. When we say that someone ought to have done something but did not, we do not normally assume that they had the moral capacity to do so. Without getting too virtue-ethicsy, it seems we are often criticizing their moral character itself. The fact that the brave and selfless have a temperament absent in others which disposes them to perform brave and selfless acts may causally explain their actions, but it does not explain them away. Similarly with the selfish or morally timid: we criticize the man who does not rescue the drowning child for fear of getting his shoes wet not because he was a morally capable person who made a bad choice, but because he is obviously morally deficient. A person with less moral capacity than another is a less moral person. To criticize their action is also to criticize their moral constitution. We are criticizing them as moral agents, and unless we fall back upon some inner citadel of personhood, there is nothing else to criticize or forgive but a person’s behaviour.
As I have argued with relation to free will and determinism:
If we take humans for what they really are, i.e. meat machines conditioned to behave in certain ways by natural selection, the free will problem becomes tractable. There is no brute mental entity making choices in a vacuum, but it is us making choices nonetheless. We are physical things (we are also mental things, but every mental thing is a physical thing, differently described), and the causal determinism of the universe flows through us, as just another part of said universe, to produce our actions. Who we are and the choices we make may be entirely predetermined by the prior state of the universe, but this does nothing to alter the fact that we act based on preferences and reasons.
We could have acted differently had we been different people with different preferences and reasons, even if we could not have been different people. It doesn’t matter how we came to be the people we are, only that we are those people and that we make choices. That’s the only freedom we are are ever going to have and I, for one, am grateful for it.