Sometimes it is created by a third party, such as a competitor's ad or a news story. The good news is that the creation of doubt is usually under the control of the seller. If you are aware of what may produce doubt, and move to eliminate those sensory cues, you can prevent the buyer from throwing up walls of doubt which may be difficult to overcome. You need to be prepared to overwhelm any doubts the customer may raise. You need to be ready to respond to doubts created by your competitors' advertising.
Doubts usually fall into two areas of concern from the buyer' s perspective.
As you can see, the buyer's conscious mind sees doubts as information-based concerns or discrepancies. They say this, but I see or know something different. Even buyers would agree that their doubts can be erased by the addition of new information. People get bamboozled all the time because they believe the explanation, even if it is farfetched. That's because doubt often cannot be simply erased by new facts.