One user offered to pay workers 25 cents to write a one-star product review on Amazon.com of three different dog training collars.
"We don't know what he's doing, but he's trying to sabotage the competition," Ipeirotis said of that user's post. After Ipeirotis released his findings last year, he said Mechanical Turk "acted rather quickly" to prevent spam-like posts on its site.
"Since then, there are still attempts and people are trying to be more careful how they describe the tasks," Iperiotis said. "They don't directly ask people to post spam, but in a very indirect way they can do that. It still exists, but I can see why it's hard to detect these cases."
Amazon could not be immediately reached for comment.
When asked if his research dissuaded him personally from trusting online reviews, Ipeirotis said no.
"Spam cannot overwhelm the market. I just trust a little bit more products that have reviews that cover diverse points of view," he said.