They collaborated with an unnamed Web site, the company did not want its involvement disclosed, on which users submit links to news articles. Readers can then comment on the articles, and they can also give up or down votes on individual comments. Each comment receives a rating calculated by subtracting negative votes from positive ones.

The experiment performed a subtle, random change on the ratings of comments submitted on the site over five months: right after each comment was made, it was given an arbitrary up or down vote, or — for a control group — left alone. Reflecting a tendency among the site’s users to provide positive feedback, about twice as many of these arbitrary initial votes were positive: 4,049 to 1,942.

The first person reading the comment was 32 percent more likely to give it an up vote if it had been already given a fake positive score. There was no change in the likelihood of subsequent negative votes. Over time, the comments with the artificial initial up vote ended with scores 25 percent higher than those in the control group.

“That is a significant change,” Dr. Aral said. “We saw how these very small signals of social influence snowballed into behaviors like herding.”

Meanwhile, comments that received an initial negative vote ended up with scores indistinguishable from those in the control group.

The Web site allows users to say whether they like or dislike other users, and the researchers found that a commenter’s friends were likely to correct the negative score while enemies did not find it worth their time to knock down a fake up vote.

The distortion of ratings through herding is not a novel concern. Reddit, a social news site that said it was not the one that participated in the study, similarly allows readers to vote comments up or down, but it also allows its moderators to hide those ratings for a certain amount of time. “Now a comment will more likely be voted on based on its merit and appeal to each user, rather than having its public perception influence its votes,” it explained when it unveiled the feature in April.