If the opening paragraph ("You want") is intended to be descriptive of my goals, rather than merely addressing some hypothetical person, I'm afraid it doesn't describe them at all accurately.
I want to buy organically produced food (in so far as I do) not in order to get certain causes invested in, but in order to get better food. I don't think it's obvious that "the cheapest food of a given quality [] is not organic" (though I'm sure it's sometimes true) and in any case I don't have ready access to an accurate food-quality-ometer and have to rely on heuristics to some extent; preferring organically produced food is one such heuristic.
(Of course it might turn out to be a rotten heuristic. That's a separate issue.)
As far as optimal philanthropy goes, I'd be shocked if marginal improvements in the gastronomic life of middle-class Westerners came within three orders of magnitude of optimality for any reasonable person, no matter how efficiently pursued. Improving the welfare of the animals eaten by middle-class Westerners might do (it depends on the priority one gives to the welfare of non-human animals, and of course "philanthropy" would no longer be quite the right word) but indeed buying organically produced food seems like an inefficient way of pursuing that goal. Anyway, philanthropy has very little to do with my organic purchasing habits.
I've been wondering whether there's any solid evidence that organic food is healthier than conventionally produced food-- the arguments I've seen on the subject have been theoretical/aesthetic.
I'm interested in anything in the range from personal stories to scientific studies, but would prefer to avoid extremely general arguments or claims that people who prefer one or the other are demonstrating character defects.
Edited to add: Thanks for the replies. I'm hoping for experiments which test the effects of food produced in various ways on organisms, especially multi-cellular organisms. It was interesting to find out that plants which have to fight off insects for themselves have more mutagens.
The mutation experiment is very cute, but it leaves out the possibility of damage that isn't related to mutation-- for example, hormonal effects. Also, if it's done on a standard bacteria acquiring the ability to make a particular nutrient, this might not be a test for mutation in general.
I realize experiments on organic vs. conventional would be difficult, especially if you're tracking human health. It would be very hard to avoid confounding factors like other lifestyle factors and income.
"Conventional food" is actually a large blob of a concept-- different pesticides, fertilizers, etc. are used on different foods at different times, so finding out what people are actually exposed to would be difficult.