Trying to find the source of a rumor is a fool's errand.
Once a rumor is in the wild, you can't unring that bell, especially in the age of e-mail chains, text messages and Twitter.
Identifying the source can be useful when defending against other forms of attack, such as published lies or a website full of propaganda. In those cases, you can identify the opposition and discredit them.
Rumors aren't identifiable. They're in the ether.
The worst rumors are about non-factual things. That doesn't mean lies. It means things you can't know.
- Future events
- Past mysteries
- Motivations and inner thoughts
I could start a rumor tomorrow saying that Justin Timberlake is sick of singing and will quit in 2012 to become an actor.
You don't need any evidence to start this rumor. It's about (1) the future and (2) another human being's inner thoughts and motivations.
It's also easy to start rumors where the target has to prove a negative, which is almost impossible.
This is why you see rumors about famous politicians and actors being accused of being gay or having an affair.
Rumors are inherently tough to squash because they have built-in defenses. Nobody can know the future. There's never any proof of another person's innermost thoughts, feeling and motivations.
You can only guess about the future, and about the secrets other people keep. This is why rumors are born and persist.