A study that tapped onto the thriving underground market for fake product reviews has found that millions upon millions of sellers rely on them, and they do so because, in the short term at least, they work, according to the Harvard Business Review.

The researchers—from UCLA and USC—found that many fake reviews are sourced from online groups where sellers recruit people to buy their product and post a good review, after which they are reimbursed via PayPal or some other platform for the purchase, and sometimes a small commission of about $5 to $10. The study found that there are over 4.5 million seller-sourced fake reviews this year alone, many times over Facebook.

The study also found that the products most likely to have fake reviews sell for between $15 to $40, are generally not name brands, and most likely came from China. On average, these products have ratings of about 4.4 over 183 reviews.

The reason sellers buy these fake reviews is because they're effective. Once a seller recruits fake reviewers, sales tend to increase by about 12.5 percent. Yet it also seems that the strategy is not as effective in the long run, as only eight weeks later, the product's rating falls by about 6.3 percent and the sales rank tends to fall by 21.5 percent, as the  one-star reviews from people who may have gotten taken in by the false ratings start piling up.