It is a somewhat common attitude to assume that the moral responsibility a person bears can be reduced by external factors. For example it is often argued that a juvenile delinquent is perhaps not as morally responsible for their actions because they were simply “raised badly” as compared to the majority of people. The idea behind this assumption is that if there are two (or more factors) that are the cause of someone’s actions, say their decisions and bad parenting, then moral responsibility should be distributed among them, and thus the person is to blame less than in the case where there were no external causes for their bad choices.
Unfortunately this idea, as nice as it sounds, ends up reducing the moral responsibility of everyone to zero. To see why consider the following: an action that a person takes is caused solely by the mental states of that person at the previous moment (excluding of course strange cases where their arms are being manipulated by puppet strings). However those mental states were in turn caused by the mental states of the immediately preceding moment, combined with information from the external world reaching the person at the same time. We can continue this process of finding causes for earlier and earlier mental states until we arrive at a situation where the mental states are caused completely by non-mental events (probably some time before the person’s birth). Now consider how we would assign moral responsibility given this analysis. Even if we assign only an infinitesimal or zero amount of responsibility to the external influences acting on a person at each moment eventually we reach a state where the person is caused completely by something that could not be considered that person, and the remaining moral responsibility ends up being assigned to that cause. Thus, in a complete analysis, the person ends up bearing no moral responsibility for their actions. Clearly this is unacceptable.
How should we think about assigning moral responsibility then? I am not going to propose a theory here (although I do have a few ideas), but it is important to note here that even if a person’s moral responsibility cannot be diminished it doesn’t mean that additional moral responsibility can’t be also assigned to factors that caused them to act in a certain way. It is contrary to every system of ethics to suppose that those who order others to commit atrocities bear none of the moral responsibility. However, no matter how much responsibility those who give the orders must bear, the responsibility of the people who did the killing is not reduced in the slightest.
One final note: although external causes cannot be said to reduce moral responsibility internal causes might be able to do so. For example you may be able to rationally claim that a person is not as morally responsible for their actions if they are insane. This is because their insanity could be considered is at least partially directly responsible for the person’s actions (unlike the causes addressed before, which were only indirectly responsible for the person’s actions).