I think there is some truth to the theory that a person will believe a statement to be true depending on who is saying it. However, I do not think it is based on the popularity of the person who is making the statement. I believe it is based both on how much trust exists between the two people and how knowledgeable the person making the statement is on the matter.
For instance, say somebody you don't get along with tells you a juicy piece of gossip about people you're not very close to. "Andy slept with Kristen," they might share with you. You don't really know Andy or Kristen well enough to know if that's true, but know Kristen has a boyfriend who she is very in love with. "No way," you say to yourself. "That would never happen." You reject the gossip as nothing more that a false rumor and the person who imparted the information as nothing more than a twit.
Later, your best friend comes up to you and delivers the same news. "Did you hear that Andy and Kristen slept together?!" she squeals. This makes you start to question the information. Maybe they did sleep together after all? Your best friend certainly wouldn't lead you astray. However, she really isn't that close to Andy or Kristen either. "How do you know?" you ask her, still unsure what to believe. She tells you that she got the information from Julie, Kristen's best friend. It must be true, then, right?
After dinner you spot Julie outside and go over to her. You ask if the rumor you've heard is true, and she confirms that it is. At this point, you finally believe it. After all, Julie is Kristen's best friend, so she would know.
My question in response to this is, What do you believe is the best course of action if people are spreading a rumor about you that is not true?