Perceived efficacy has an enormous impact on people's willingness to provide aid. Building on a series of psychological studies, the concept of pseudoinefficacy refers to the idea that people are less willing to help one person when they are made aware of the broader scope of people in need that they are not helping.
People who choose to provide aid are motivated by a "warm glow" associated with having helped a person in need. Becoming aware that there are others who are not being helped makes any effort to help feel less good (fast thinking) or like a "drop in a bucket" (slow thinking).
These feelings are irrational. One life saved is a life saved, even if other lives are not saved.
"Pseudoinefficacy: negative feelings from children who cannot be helped reduce warm glow for children who can be helped" by Daniel Västfjäll, Paul Slovic, and Marcus Mayorga. Frontiers in Psychology, 2015.