I'm sure there will be a lot of discussion here about the accuracy of the 20% figure, but that actually doesn't seem as important as the other findings from the research this article cites ("Fake It Till You Make It:
Reputation, Competition, and Yelp Review Fraud", http://people.hbs.edu/mluca/FakeItTillYouMakeIt.pdf).
The interesting points from the research include:
1) Low ratings increase incentives for positive review fraud, and high ratings decrease them.
2) Having more reviews reduces incentives for positive review fraud.
I would call all of those points extremely obvious and hardly worth a whole study.
If you have low ratings, of course you're tempted to get some fake positive reviews. If you have a lot of reviews, adding a few positive ones will not affect the overall score much--it's a drop in the bucket. Chain restaurants, well, everyone knows what McDonald's is, what would they gain from posting fake reviews?
Businesses that claim their Yelp page have more interest in Yelp? Shouldn't it be the other way around, businesses who have decided they're interested in Yelp will claim their page? Claiming your Yelp page is a sign that you care; businesses that do not care about Yelp will not commit review fraud.
Who else would post negative fake reviews but competitors? I guess reputation management companies could post negative fake reviews in an attempt to drum up business.
An increase in competition encourages more negative fake reviews? I mean, if a business has no competitors, who has an interest in posting fake reviews? This ties to #5, where competitors are the ones who post negative reviews. More competitors, more fraud.
It's nice that these points seem obvious to you, but until you have data and a sound statistical analysis, people won't have any reason to believe them. Lots of "obvious things" turn out to be true, but lots turn out to be false too.
The interesting points from the research include:
1) Low ratings increase incentives for positive review fraud, and high ratings decrease them.
2) Having more reviews reduces incentives for positive review fraud.
3) Chain restaurants leave fewer positive fake reviews.
4) Businesses who have "claimed" their Yelp page have more interest in Yelp and therefore are suspected as being more likely to commit review fraud.
5) Negative fake reviews are likely posted by competitors, and show little correlation to changes in average ratings over time.
6) An increase in competition encourages negative review fraud.