Ben Franklin discovered that a person who has done someone a favor is more likely to do that person another favor than they would be had they received a favor. Or, as Franklin put it: “He that has once done you a Kindness will be more ready to do you another, than he whom you yourself have obliged.” This can be exploited to gain your favor.
But first, let's explore a little about why this works.
Cognitive dissonance is a state of tension that occurs whenever a person holds two cognitions (ideas, beliefs, attitudes, or opinions) that are psychologically inconsistent. Dissonance produces an uncomfortable mental state the mind needs to resolve. In resolving dissonance our minds trend towards self-justification which makes it hard to admit mistakes.
Dissonance is a very powerful effect. In the case of the Ben Franklin effect, the dissonance is caused by the subjects' negative attitude to the other person contrasted with the knowledge they did that person a favor. The easiest way to rationalize why we did someone a favor is to say “that person is not so bad after all.”
In his autobiography, Franklin explains how he curried favor by manipulating a rival legislator when he served in the Pennsylvania legislature in the 18th century:
“Having heard that he had in his library a certain very scarce and curious book, I wrote a note to him, expressing my desire of perusing that book, and requesting he would do me the favour of lending it to me for a few days. He sent it immediately, and I return'd it in about a week with another note, expressing strongly my sense of the favour. When we next met in the House, he spoke to me (which he had never done before), and with great civility; and he ever after manifested a readiness to serve me on all occasions, so that we became great friends, and our friendship continued to his death.”