Who's writing all those nasty online reviews—the ones denouncing everything from defective pants to soggy veal piccata? You'll never guess, say two academics who have exhaustively researched the question. It's not a few cranky customers, they say. Nor is it some competitor hiding under a pseudonym or paying surrogates to slam a business rival.No, say Eric Anderson and Duncan Simester—professors of marketing at Northwestern University's Kellogg School of Management and MIT's Sloan School of Management, respectively—the writers of negative reviews tend to be the best customers of the business being slammed. By "best" they mean the most loyal and most valuable (customers who spend the most). That's extraordinary finding #1.