While true, this runs into the main caveat of the "only support one charity" advice- you have to be donating an amount small enough to not change the marginal value of a dollar. When that's true, dollars and utils are linearly related, meaning there's no direct benefit for specialization.
Also, for existential risk the negative correlation hedge doesn't matter. Hedging your bets only pays off when one bet succeeds and another bet fails, but with x-risks, if a single bet fails all of the bets stop mattering. So you should figure out which x-risk gives you the strongest returns when you fight it, and devote all your resources to that until the marginal value drops below another x-risk.
Now, there is a strong argument for diversifying due to ignorance- if you think that A will reduce risk by 5+/-1 and B will reduce risk by 4+/-1.5, then you should give 71% of your money to A and 29% to B.
To illustrate what I meant, if you are giving to charities that aim to cure a fatal disease that you happen to have - then that means you have an increased risk of ruin if your donations don't help - broadly similar to the one that investors diversify their portfolio to help prevent if their individual investments don't pay off.
Of course, that is not selfless altruism - but it is still giving money to charities with the aim of helping the charities to meet their goals - rather than for signalling purposes.
Related: Purchase Fuzzies and Utilons Separately
We genuinely want to do good in the world; but also, we want to feel as if we're doing good, via heuristics that have been hammered into our brains over the course of our social evolution. The interaction between these impulses (in areas like scope insensitivity, refusal to quantify sacred values, etc.) can lead to massive diminution of charitable impact, and can also suck the fun out of the whole process. Even if it's much better to write a big check at the end of the year to the charity with the greatest expected impact than it is to take off work every Thursday afternoon and volunteer at the pet pound, it sure doesn't feel as rewarding. And of course, we're very good at finding excuses to stop doing costly things that don't feel rewarding, or at least to put them off.
But if there's one thing I've learned here, it's that lamenting our irrationality should wait until one's properly searched for a good hack. And I think I've found one.
Not just that, but I've tested it out for you already.
This summer, I had just gone through the usual experience of being asked for money for a nice but inefficient cause, turning them down, and feeling a bit bad about it. I made a mental note to donate some money to a more efficient cause, but worried that I'd forget about it; it's too much work to make a bunch of small donations over the year (plus, if done by credit card, the fees take a bigger cut that way) and there's no way I'd remember that day at the end of the year.
Unless, that is, I found some way to keep track of it.
So I made up several jars with the names of charities I found efficient (SIAI and VillageReach) and kept a bunch of poker chips near them. Starting then, whenever I felt like doing a good deed (and especially if I'd passed up an opportunity to do a less efficient one), I'd take a chip of an appropriate value and toss it in the jar of my choice. I have to say, this gave me much more in the way of warm fuzzies than if I'd just waited and made up a number at the end of the year.
And now I've added up and made my contributions: $1,370 to SIAI and $566 to VillageReach.
A couple of notes:
Let me know if you start trying this out, or if you have any suggested improvements on it. In any case, may your altruism be effective and full of fuzzies!
ADDED 12/26/13: I've continued to use this habit, and I still totally endorse it! A few addenda: