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They noticed that the choices made by friends also affected the user's own choice. 

For example, they observed that if people reviewed a restaurant positively and this person's friends saw the review, they gave the restaurant a more positive review than if they had not seen the friend's review. On the other hand, a friend's negative review of a restaurant did not have the same impact on their friends' reviews.

This has practical consequences. If a company wants to receive positive reviews for a product, it should start by encouraging people who like the product to evaluate it for others. The situation is also influenced by the difficulty in eliminating the effects of fraud. If a company has written positive assessments of its own product, the impact of these is easily retained in future reviews even if the original positive fake message is later removed from the review.

Watch Aral's talk .