University of Oregon psychologist Paul Slovic asked subjects for donations to save a little girl from starvation. To one group he gave no other information; to the other group he added that this girl was one of millions of other starving people. Logically, that extra bit of information should make no difference, since the girl being saved is the same.
But as one of my mentors likes to say, human beings are psychological and not logical creatures. Case in point: subjects in the second group donated about half as much money as those in the first group.