Adding another dimension to the research, de Langhe and cohorts write that user reviews do a poor job of predicting resale value of used products, another way of determining the quality.
"Products with better reliability and performance retain more of their value over time," said de Langhe. "If average user ratings reflect objective quality, they should correlate positively with resale values. The fact that they don't casts more doubt on the validity of user ratings."
In essence, nothing stops your friend who puts ketchup on macaroni and cheese from adding Yelp ratings daily to complain about how a restaurant's mac and cheese doesn't have ketchup on it. Those ratings, over time, influence other consumers' decisions.