Customer reviews are a common feature of many online shopping sites. Hearing the opinions and experiences of other customers can be extremely persuasive. The mechanism underlying customer reviews is social proof. Cialdini defined the principle of social proof as 'the tendency to view a behaviour as correct when we see others perform it'.
Social proof can be seen in an experiment by Milgram, Bickman and Berkowitz (1969). The researchers composed crowds of varying sizes (1,2,3,5,10 or 15) and instructed them to look up to a 6th floor window for 60 seconds. The researchers measured the number of pedestrians who stopped alongside the crowd and looked upwards.
Fig 1. shows that the percentage of pedestrians who stopped and looked up, increased as the size of the crowd increased.
The pedestrians assumed that since a large crowd of people were looking upwards, there must be a good reason for it and followed this behaviour. Similarly, potential customers reading a review may think 'other people have bought this product, so I should too'.
Cialdini, R. B. (2009). Influence: Science and practice.Boston: Pearson Education
Milgram, S., Bickman, L., & Berkowitz, L. (1969). Note on the drawing power of crowds of different size. Journal of Personality and Social Psychology, 13(2), 79-82.