Linked from
The 9 pages that link to Credibility, each with the reason it gives.
DeterrenceRelated: A deterrent threat matters only if its target believes it will be carried out.
HyperboleRelated: Repeated or implausible exaggeration can make a speaker seem less trustworthy.
DistrustRelated: Judgments of credibility provide a common basis for trusting or doubting testimony.
Hard powerRelated: A threat has little coercive effect if its target doubts the state will act.