It’s a commonplace saying in NLP that “most people have a ‘three-time convincer'” – in other words, people need to experience three examples of something to be convinced.
Now there’s some research evidence to back this up, from a study into what people perceive as a run or ‘streak’ of luck in sport, gambling, or playing the markets:
… Kurt Carlson and Suzanne Shu report that the key moment we perceive a streak as having occurred is after three repeats – what they call the ‘rule of three’. In other words, we don’t read meaning into a repeat of two, and we don’t read any additional meaning into streaks of more than three.
In one study, students were asked to decide how much fictional inheritance to invest in a stock. After hearing the stock had risen for one day or for two consecutive days, there was little increase in the amount they chose to invest. The largest jump in the students’ investment decision came after they learned the stock had risen for three consecutive days. By contrast, hearing that the stock had risen for four, five or even six consecutive days didn’t make any further impact on their decision making.
(The research report comes from the BPS Research Digest, which is essential reading for anyone interested in how the mind works)
Implications: if you want to convince someone of something, you have a better chance if you give them at least three examples. And if you want to modify your self image as being a certain kind of person (one who exercises regularly, for example), you will probably need at least three examples before you start to really believe it.
Of course, as Shelle Rose Charvet points out in the excellent Words That Change Minds, there are other variants of the “Convincer Demonstration Filter”. Some people are instantly convinced and believe things straight away, others need two or four demonstrations, some need a certain period of time to make their mind up, and some are never convinced!