this is covered as one of the common bias effects from social psychology, I think you'll find it in any social psych textbook.
a newspaper article reporting that 9 out of 10 brand "A" car owners love ther car is less persuasive than your office neighbor telling you that he's nr. 10 - buy brand "B." etc.
operant psychology also voices opinions about this effect. your office neigbor is a more direct stimulus than a newspaper article.