If you search for the millennial makeup brand Glossier on YouTube, one of the first results likely to appear is from Olivia Jade, a fashion and beauty vlogger with over one million subscribers. Her 2017 video " First Impression & Review of Glossier Makeup" has garnered over 630,000 views. In the video's description, Jade writes "this video is not sponsored!" She fails to mention, though, that a list of products featured in the video include a series of affiliate links produced by RewardStyle, an agency that creates affiliate marketing campaigns for influencers like Jade. If a viewer clicks on a RewardStyle link and buys a product like Glossier's Hydrating Moisturizer or Boy Brow Gel, Jade likely gets a cut of the sale.

Jade is far from the only influencer to neglect to disclose affiliate marketing relationships according to Federal Trade Commission guidelines. New research released from Princeton University Monday indicates that the vast majority of similar marketing set-ups go undisclosed by influencers on platforms like YouTube and Pinterest.

In a paper to be presented at the 2018 IEEE Workshop on Technology and Consumer Protection in May, Princeton's Arunesh Mathur, Arvind Narayanan, and Marshini Chetty analyzed a representative sample of over 500,000 YouTube videos and over 2.1 million unique Pinterest pins collected from August to September 2017. Of those, 3,472 videos and 18,237 pins had affiliate links. And of that subset, researchers found that only 10 percent of YouTube videos and seven percent of Pinterest pins contained any written disclosure.

The majority of YouTube and Pinterest influencers are likely making a profit off their product reviews—even without direct corporate sponsorship—without disclosing that fact to users.

The vast majority of disclosures that the Princeton researchers did find don't even abide by FTC guidelines. In 2013, the agency began requiring that affiliate links embedded within product reviews include a disclosure. In the current version of the guidelines, bloggers are required to include more than just the phrase “affiliate link,” because readers and viewers may not know what the term means. The FTC instead recommends that bloggers use a short explanatory phrase, like “I make a commission through purchases made through this link.”

Of the few disclosures the researchers found, most merely included phrases like "affiliate links may be present above." Disclosures that contained an actual explanation of what an affiliate link is only accounted for a tiny fraction of the YouTube videos and Pinterest pins the researchers looked at. That means the majority of YouTube and Pinterest influencers are likely making a profit off their product reviews—even without direct corporate sponsorship—without disclosing that fact to users. (Like most online publications, WIRED also participates in affiliate marketing).

"​Disclosures are important so users can give—in their minds—appropriate weightage to content creators’ endorsements," says Arunesh Mathur, a computer science graduate student at Princeton and the lead author of the paper. He says that his study's findings likely don't represent all undisclosed affiliate marketing campaigns on Pinterest and YouTube, because the researchers didn't take into consideration other forms that don't include links, like coupon codes.

The study also only included descriptions written in English, and couldn't account for other kinds of undisclosed marketing relationships, like when an influencer is given a product for free, or paid a fee behind the scenes to promote it. In fairness, the research also doesn't take into account instances in which a vlogger discloses the affiliate marketing campaign in the video itself, or within the image on Pinterest. Mathur, though, doesn't believe most disclosures take that form. "We're fairly confident that only a tiny fraction of content creators disclose affiliate links at places other than the description," he says.

The Princeton research underscores how murky the world of product reviews on YouTube and Pinterest really is, where it's often impossible to definitively know how an influencer profits from a post. And since affiliate links tend to be used by more popular accounts, recommendations and search engines are more likely to surface posts that have them.