A recent study provides ample evidence of something we’ve suspected all along: when offering reasons in an attempt to persuade, three reasons work best.
Researchers at Georgetown University and UCLA looked at six scenarios where reasons or claims were offered to persuade:
A breakfast cereal
An old friend reuniting with a former boyfriend
A new brand of shampoo
An ice cream shop
For each of these six scenarios, one, two, three, four, five, and six reasons were offered in support of the persuasion attempt.
The study concluded that when the audience suspects that persuasion is a motive (which is always the case with business audiences), persuasiveness peaks with three positive claims, and falls off with four or more claims. As the number of claims increases from one to three, targets find the additional information useful. But once you hit four, you trigger their skepticism and your persuasiveness diminishes significantly.
Lesson: Always offer three reasons in support of your proposal, idea, or argument. If you only have two, think of another. If you have four or more, cut the excess out. Three is the magic number.
See Kurt A. Carlson and Suzanne B. Shu, When Three Charms but Four Alarms: Identifying the Optimal Number of Claims in Persuasion Settings, at http://papers.ssrn.com/sol3/papers.cfm?abstract_id=2277117