Eliezer has written a post (ages ago) which discussed a bias when it comes to contributions to charities. Fragments that I can recall include considering the motivation for participating in altruistic efforts in a tribal situation, where having your opinion taking seriously is half the point of participation. This is in contrast to donating 'just because you want thing X to happen'. There is a preference to 'start your own effort, do it yourself' even when that would be less efficient than donating to an existing charity.

I am unable to find the post in question - I think it is distinct from 'the unit of caring'. It would be much appreciated if someone who knows the right keywords could throw me a link!