What is the Ben Franklin effect?

The Ben Franklin effect is a psychological phenomenon where people start to like someone more after they do a favor for that person. This is based on the idea that people want to maintain a positive self-image, so when we do something for someone else, we justify our actions by believing that we must like that person. The term comes from a story where Benjamin Franklin softened the opinion of an adversary by asking to borrow a rare book from him. By doing so, Franklin's adversary began to feel more positively towards him.