Just pointing out that the idea that giving is good for the soul (i.e. "makes one happy" in materialist parlance) is an ancient idea, already much told to people. Of course, you can't talk about souls to materialists, but if you find that "science says it will make you happy" persuades them, why not?

Someone should do the experiments. ("And not only that, but science says that you'll give even more if we tell you it will make you happy!" There's got to be an SMBC in that.)

Experiment rules, but in advance of that I can think of ways it could go wrong, which might suggest further experiments. One is the burnout I mentioned. Another is this. If the actual mechanism of giving causing happiness is:

think a cause is worthy --> give to it --> satisfaction

then what happens in the situation:

indifferent to a cause --> give only because "science says" --> ??

Are you really buying happiness with money, or are you buying it by directing that money to something you already thought worth spending money on? There are issues of reflexive decision theory here.