Amazon review system is one of the keys to their success.  They work hard on making sure reviews published on Amazon can be trusted and this means they try to identify and review what they consider “biased” reviews.  This article takes a look on how they determine what reviews are biased by finding a review that might be biased towards the author.  Obvious things are same address as the author, same IP as the author, multiple reviews from the same person… but there are other less obvious clues as well.