A big chunk of those “likes,” “retweets,” and “followers” lighting up your Twitter account may not be coming from human hands. According to new research from the University of Southern California and Indiana University, up to 15 percent of Twitter accounts are in fact bots rather than people.

The research could be troubling news for Twitter, which has struggled to grow its user base in the face of growing competition from Facebook, Instagram, Snapchat and others.

Researchers at USC used more than one thousand features to identify bot accounts on Twitter, in categories including friends, tweet content and sentiment, and time between tweets. Using that framework, researchers wrote that “our estimates suggest that between 9% and 15% of active Twitter accounts are bots.”

Since Twitter currently has 319 million monthly active users, that translates to nearly 48 million bot accounts, using USC’s high-end estimate.

The report goes on to say that complex bots could have shown up as humans in their model, “making even the 15% figure a conservative estimate.”

At 15 percent, the evaluation is far greater than Twitter’s own estimates. In a filing with the SEC last month, Twitter said that up to 8.5 percent of all active accounts contacted Twitter’s servers “…without any discernible additional user-initiated action.”

Since that equates to roughly 20 million more bot accounts than Twitter’s own assessment, that could be an issue in light of analyst concerns about user growth. In a recent research report, Nomura Instinet analysts wrote that “Twitter’s revenue growth has slowed to the mid-single digits, as the platform has struggled to attract new users over the past year…”

A Twitter spokesperson said that while bots often have negative connotations, “many bot accounts are extremely beneficial, like those that automatically alert people of natural disasters…or from customer service points of view.”

USC’s researchers also highlight the benefits of some bots, writing, “many social bots perform useful functions, such as dissemination of news and publications…”

But the USC report also points to the downside of bots, saying, “there is a growing record of malicious applications of social bots. Some emulate human behaviour to manufacture fake grassroots political support… [and] promote terrorist propaganda and recruitment.”

Twitter currently has a number of ways to report violations, including impersonation accounts and spam. A number of services also exist which claim to be able to audit followers and identify fake accounts.