Again, you ask lots of great questions. I will turn to the question of whether positive or negative appeals for charity are stronger motivators of donation. I believe there is some evidence for both sides. Deborah Small and Nicole Verocchi have done some work showing that charitable appeals, for example, that encourage donations to children elicit higher donations if the child depicted has a sad facial expression vs. a smile because this signals the child is truly in need and it evokes people’s sympathy. Now whether charities have actually put that work into action is less clear to me. On the other hand, it seems pretty dicey to play around with negative emotion because there also exists work showing that the negative emotion of guilt, for example, can at times have backfire effects on moral behavior. Also, you're opening the floodgates if you want to talk about whether people are more motivated by rewards vs. punishments to engage in charitable giving—again, a larger topic for another time.
To me, the real expert on this topic is John List, an economist, who has done a lot of nice field experiments on charitable giving. Amongst other things, he has found that charitable donations increase when the asking agent is an attractive woman, a finding not necessarily relevant here but an intriguing one that always makes me curious about the mechanism at play. Probably the work of his that pertains most to the questions we have been discussing is research showing that previous donors are more likely to give than people who are asked to give for the first time, which supports the idea of moral identity leading to moral consistency. But there is a wrinkle to this finding as well, which is that it depends on how the previous donors come to the charity in the first place--they have to be attracted to the charity itself to begin with (rather than to some extraneous factor, like receiving a gift for their contribution). Again, overall I think this supports the moral consistency idea, which is that people who feel truly committed to a cause internalize that as part of their identity and are then more likely to behave charitably for subsequent requests.
But the more interesting case for me, still, is when we see acts of charity leading to people slacking off in terms of other or subsequent moral obligations. This effect is less well established in the field, but I know you have some ideas on when and where we might find it. Let’s hear them.
Image courtesy of Bertel Thorvaldsen via Wikimedia Commons