Saddly I still find many, many people arguing that "altruism" or "selflessness" just can't exist, that everyone is purely selfish, and that helping others is only done because either you accept them to pay back later on (IPD-like) or because it makes you feel good to do so.

I tried many arguments, from opportunity cost (yes, giving to charity may give a "warm fuzzy" as Eliezer says, but spending the same money in buying a video game, going to a concert or eating yummy food can easily give more happiness), to exceptional situations (an atheist (so you can't invoke fear of hell) from the Resistance withstanding torture to not betray his friends), but they always manage to dodge the issue and find pseudo-arguments like "they still do it only because of fear of shame".

So I ended up trying to imagine hypothetical situations like "Imagine aliens kidnap you, and offer you a choice between pressing Blue Button or Red Button. If you press Blue Button, you'll forget anything about aliens, awake next day in perfect health, find a winning lottery ticket in your mailbox, but aliens will destroy the Earth once you're dead from natural death. If you press Red Button, aliens will offer to Earth a cure for cancer, aids, ... but they'll torture you for months and then kill you. Do you say no human would ever press the Red Button ?" But I guess that even then they'll say "but the shame felt while pressing the Blue Button will be too high".

And anyway "escalating" the conflict at this point doesn't feel a "clean" answer to me. So I'm still looking for a more clever way of making people understand that humans are more complicated and you can't explain all of "altruism" by just guilt feelings and warm fuzzies.