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KANSAS CITY, Mo. –Social review sites started out as a great way to share tips about a hip new clothing store or a hot restaurant. In fact, those reviews have become so important that business owners will got to great lengths, even break the law, to manipulate them.

Whether you are in Paris, San Francisco or just downtown, many of us now jump online to find out where to eat and where to shop.

And businesses know that, so much so that some cheat, posting fake online reviews about their establishment and even their competitions.

“There is a little bit of an arms race going on here,” said Harvard Professor Michael Luca who conducted a study of posts on the website “One restaurant will game the system and then a lot of restaurants will feel the need to do this.”

Yelp estimates more than 20 percent of its reviews are fake, but Luca believes the figure is much higher. His research proved what many had long assumed: There’s an increase in the number of fake negative reviews, when a new business opens in an already crowded market. And restaurants struggling to stay afloat show an increase in fake positive reviews.

Yelp has set up an elaborate system to filter out the fakers, but Luca believes many escape the filter. In fact an entire industry has emerged offering business reviews for a price.

Courthouse Cafe in Independence, Mo., has even received phone calls from fake review writers trying to drum up business.

“They ask if we want them to come in the establishment and write a review for us,” General Manager Mary Bauer said. “We just decline.”

Bauer takes online reviews seriously. She even reads them to her staff. “What’s the good, what’s the bad. It’s more of a working tool,” she said.

That’s true also at Remedy, a restaurant in Kansas City’s Waldo neighborhood.

“When they mention a server, I’ll say `Hey you did a great job on that table. Keep up the good work,'” said Manager Stewart Hemmer.

But Hemmer said he is no fan of most social review sites because people can post anonymously.

“It’s an open forum for people to vent,” Hemmer said. “It’s very passive aggressive.”

So where does that leave us, the public, when trying to figure out whether to trust an online review? Here’s what the experts recommend:

Tip #1
Long reviews are more trustworthy than short ones. Anyone moved enough to write a review, whether angry or thrilled, will usually write more than just a line or two.

Tip #2
Frequent reviewers are more reliable than someone who has never posted before. That new poster might be a fake.

Tip #3
Compare the same business on multiple review sites, including professional reviews such as those written by a newspaper’s restaurant reviewer.