For example, one might judge that it is bad to kill an innocent person even if his vital organs could be used to save five others who desperately need transplants. Here, arguably, we feel cumulatively more empathy for the five people in need than for the one healthy person, but our moral judgment does not track that empathetic response.

The contrary, it is easier to empathize with one person than five:

The reason human beings seem to care so little about mass suffering and death is precisely because the suffering is happening on a mass scale. The brain is simply not very good at grasping the implications of mass suffering.