from DPReview :
"Online product reviews are a huge part of our lives these days, and often, they're the nearest thing we get to a 'hands on experience' before we buy something. Extremely positive or negative reviews can greatly alter the perception of a product in the mind of an undecided customer.
So how do we know that the reviews we're reading are honest? The authors of an MIT study entitled 'Deceptive Reviews: The Influential Tail' have looked at several sites which feature customer reviews, including Amazon, but focused on an unnamed private label apparel retailer. Crucially, this unnamed retailer does not link reviews to a purchase of that item - i.e., you can write a review of something even if you didn't buy it.
What they found is that 'approximately 5% of the product reviews are written by customers for whom there is no record they have purchased the item . These reviews are significantly more negative on average than the other 95% of the reviews for which there is a record that the customer previously purchased the item"
(at the bottom of the article there is a link to the full report.)
The reason I posted this is because a pet peeve of mine is seeing people(including here) commenting , usually negatively, on items that they clearly have no idea whatsoever of what the item even looks like never mind having any hands on experience with it.
Anyway the study addresses the issue of why people feel the need to do that.