http://www.womma.org/index.htmexcerpt dealing with this particular issue:
2. The Honesty ROI: Honesty of Relationship, Opinion, and Identity
Honesty of Relationship
We practice openness about the relationship between consumers, advocates, and marketers. We encourage word of mouth advocates to disclose their relationship with marketers in their communications with other consumers. We don't tell them specifically what to say, but we do instruct them to be open and honest about any relationship with a marketer and about any products or incentives that they may have received.
We stand against shill and undercover marketing, whereby people are paid to make recommendations without disclosing their relationship with the marketer.
We comply with FTC regulations that state: "When there exists a connection between the endorser and the seller of the advertised product which might materially affect the weight or credibility of the endorsement (i.e., the connection is not reasonably expected by the audience) such connection must be fully disclosed."
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http://womma.org/ethicscode.htm