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.

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.