Is there a particular philosophical discipline that deals with large numbers of people doing something innocuous, but having a deleterious effect on a much smaller number of people? If so, does it have a name? Like blame-proration, guilt-apportionment, or anything? Thanks!

Perhaps an example would help, but I think I have the idea. We might want to start by modifying your description a bit. You wrote of large numbers of people doing something innocuous but having a bad effect on a small number of people. If you think about it, however, that means the word "innocuous" isn't really right. And so I'm guessing you have something like this in mind: there's a certain sort of action (call it X-ing) that large numbers of people perform that has something like this handful of features. First, it doesn't harm most people at all. Second, though X-ing is potentially harmful to some people, the harm would be minimal or maybe even non-existent if only a few people X-ed, and only occasionally. Third, however, enough people actually do X that it causes palpable harm to the small minority. And given your suggested terms ("blame-proration," "guilt-apportionment") I take your question to be about just how culpable the people who X actually are.

If that's right, it's a nice question. The broad discipline is ethics, though giving a name to the right subdivision is a bit tricky (especially for someone like me who's interested in the field but doesn't work in it). We're not up in the stratosphere of meta-ethics where people work if they're interested in whether there are objective moral truths, for instance. We're also not at the level of normative ethics where people propose and/or defend frameworks such as utilitarianism or virtue ethics. But we're also not as close to the ground as work in applied ethics. Your issue is theoretical, with both conceptual and normative components, and with potential implications for practical or applied ethics. There's actually a lot of work of this sort in the field of ethics. If I were going to tackle this sort of question, I'd start by assembling some examples to clarify the issues. I wouldn't restrict the examples to cases that are quite as focussed as the question you raise. For instance: I'd also look at cases in which the number of people potentially harmed may not be small, but where the individual actions, taken one at a time, don't seem objectionable. If I drive to work rather than taking the bus, I've increased my carbon footprint—not by a lot in absolute terms, and it's not as though there are no reasons for taking my car rather than the bus. (I'll get to my office earlier and may get more work done, for instance.) But if enough people drive rather than take the bus, the cumulative effect is significant. Your problem is even more specific, but you can see that there's an important connection. And the sort of problem I've identified is one that's been widely discussed. (It's a close cousin of what's sometimes called "the problem of the commons.")

So anyway: the broad discipline is ethics, and the question has both theoretical and practical elements. It's also an interesting issue to think about. Perhaps other panelists who actually work in ethics will have more to say.