Amazon.com is tightening controls on "incentivized reviews," in which customers receive free or discounted goods in exchange for writing product critiques that many shoppers rely on when making online purchases. Amazon previously allowed such reviews to help new products pick up traction on its marketplace, as long as the person writing the review disclosed the discount.

The e-commerce giant now will allow such reviews only through a program it monitors, called Amazon Vine, which was introduced in 2007. The company selects the customers conducting the reviews on promotional products, so there is no direct link to the sellers or expectation of a positive review.

"Amazon -- not the vendor or seller -- identifies and invites trusted and helpful reviewers on Amazon to post opinions about new and pre-release products; we do not incentivize positive star ratings, attempt to influence the content of reviews, or even require a review to be written; and we limit the total number of Vine reviews that we display for each product," Chee Chew, Amazon’s vice president of customer experience, wrote in a blog post about the change.

Amazon last year filed lawsuits against more than 1,000 unidentified people offering to write fake product reviews in exchange for payment as well as sites that sold fake reviews.