Valve has taken to its forums to announce that it has changed the way Steam reviews work. Those who redeem a code for a title will no longer be able to rate it using the service.

“The review score has also become a point of fixation for many developers, to the point where some developers are willing to employ deceptive tactics to generate a more positive review score,” Valve wrote in the post.

“An analysis of games across Rocket League Steam CD Key shows that at least 160 titles have a substantially greater percentage of positive reviews by users that activated the product with a CD key, compared to customers that purchased the game directly on Steam.

“There are, of course, legitimate reasons why this could be true for a game … But in many cases, the abuse is clear and obvious, such as duplicated and/or generated reviews in large batches, or reviews from accounts linked to the developer.”

The company pointed to reviews of Batman: Arkham Knight as evidence; 49% of paying players gave it a positive rating, compared to 74% of key activating players.