Amazon updated its community guidelines today to prohibit so-called "incentivized reviews," which are customer reviews of a product that was received for free or at a discount in exchange for an online write-up. The company says these types of reviews make up a tiny fraction of overall Amazon.com reviews. Yet studies have shown that incentivized reviewers are less likely to give products negative feedback and review hundreds of products on average, potentially affecting the overall sales performance of otherwise mediocre items.

The company says it will still support incentivized reviews handled through its Vine program, which was set up in 2007 to offer a more bias-free customer review system. Vine lets retailers pay Amazon a fee to distribute its products to trusted reviewers, who never deal with the maker of the product itself. The Vine program does not incentivize positive coverage and Amazon limits the number of Vine reviews it displays for any given item. One exception to this new rule is the book category, as Amazon does not want to prohibit the industry standard of sending out advance review copies of new novels and non-fiction.

Amazon has often struggled to maintain the integrity of its customer review system, which millions of consumers rely on to make smart purchase decisions and avoid faulty or substandard products. In some cases, Amazon has aggressively gone after companies that abuse its system and even sued individuals who offered reviews in exchange for cash on freelance job sites.

Over time, Amazon hopes to alter its Vine system to help improve its trustworthiness, but Chee Chew, Amazon’s vice president of customer experience, says the company is not ready to disclose details on that front quite yet. Meanwhile, the company will continue to take legal action against those who attempt to manipulate its review system for any purpose.