Unexpectedness can increase persuasion and it can also decrease it. Many authors suggest that unexpectedness and persuasion are inseparable or at least unexpectedness is one of the cornerstones in persuasion. Heath brothers, for example, make unexpectedness second element in their SUCCESs formula (see their bestselling Made to Stick). Overall, this is solid advice, but it can also backfire.
Research indeed shows that unexpectedness can boost persuasion. The reason is that unexpected message surprises your audience; the more surprised they are the more attention they pay; the more attention they pay the more likely it is that they will mentally digest the message and be swayed by it. In essence, people are more likely to meditate on your message if it surprises them.
But here is a critical requirement: the message must be based on strong arguments. If not, unexpectedness will backfire because people, being more involved and processing your message more carefully, will be more likely to spot any flaws.
- Richard E. Petty, John T. Cacioppo, Rachel Goldman, “Personal Involvement as a Determinant of Argument-Based Persuasion,” Journal of Personality and Social Psychology, Vol. 41, 847-55 (1981).
- Richard E. Petty and John T. Cacioppo, Communication and Persuasion: Central and Peripheral Routes to Persuasion (1986).