"As most readers will know by now, if you're donating to a charity, it doesn't make sense to spread your donations across several charities. You'll want to pick the charity where your money does the most good, and then donate as much as possible to that one."

This is not necessarily true. For some charities, the marginal utility is very high up to, say, the first $10 million. After that point, returns on each dollar donated might start to rapidly decline.

Admittedly this is mainly a problem for larger donors, though.