At a time when Twitter Inc. (TWTR) is suffering from a slowing user base, a new report out of the University of Southern California and Indiana University shows that perhaps as much as 15 percent of Twitter accounts are bots instead of people doing the tweeting and retweeting.

Relying on more than a thousand different criteria to identify if an account is a bot on Twitter, researchers found that 9% to 15% percent of the accounts aren’t humans. Given that Twitter has 319 million monthly active users, 15% means 48 million accounts are actually bots liking, retweeting and following companies and users of the social media network. Some of the categories researchers looked at to determine if an account was real or a bot included friends, the content tweeted and retweeted, time between tweets and sentiment.

Social Media Makes It Easy for Bots

“Social media make it easy for accounts controlled by hybrid or automated approaches to create content and interact with other accounts,” the researchers at the University of California wrote in their report. “Increasing evidence suggests that a growing amount of social media content is generated by autonomous entities known as social bots.” The researchers noted the 15% figure could prove too conservative since bots are getting much more complex and may have been identified as humans in their own research.  In February Twitter said as much as 8.5% of its active accounts may be bots.  (See also: Report: Twitter Lacks Innovation.)

The new research not only implies more Twitter accounts are bots than Twitter disclosed but that its struggling user base growth may be worse than thought. For its fourth quarter ended in December, Twitter reported adjusted earnings of 16 cents per share on revenue of $717 million, which beat Wall Street's expectations for earnings per share of 12 cents but missed revenue forecasts of $740.14 million, according to Thomson Reuters. Aside from missing on the top line, this amounts to less than 1% revenue growth, and even the beat on the bottom line translates to flat growth year over year. Twitter's revenue was hurt by weak advertising sales, totaling just $638 million, marking a slight year-over-year decline.

The revelation of bots controlling as much as 15% of Twitter accounts also comes at a time when Twitter and Facebook Inc. (FB) are trying to improve their reputations in an era of fake news and "alternative facts." With both getting criticized for helping spread fake news during the run-up to the 2016 presidential election, Twitter and Facebook have been stepping up efforts to combat that and abusive behavior. Last month, Twitter quietly rolled out a new feature in which users who are abusive are put in a digital timeout where their account is limited for a temporary period of time. Meanwhile, Facebook has started tagging content that may not be true, dubbing it "disputed." (See also: Twitter Gives Digital Timeout for Bad Behavior.)