Here are some of the reasons why these ads don't work according to the study.
The celebrity confuses the message.
Lance Armstrong doesn't really have a lot to do with Radio Shack and the spot hasn't done a great job of connecting him to the premise or the brand.
The celebrity is polarizing or actively disliked.
Tiger Woods, Brett Favre, Snookie, Rod Blagojevich, Sarah Jessica Parker, The Donald. The list is endless and obvious. Why would I buy from any of these people?
The ad is boring.
Just because you put a celebrity in an ad, doesn't make it interesting. The cosmetics industry is especially guilty of this.
That's not to say that celebrities never work.
The Betty White spot for Snickers scored highly. As does Troy Polamalu for Head and Shoulders. These commercials don't use celebrities as a substitute for the idea, they are integral to the idea. That's why they work. They're relevant. They're memorable. And they make a real point about the product.
Something all good advertising is supposed to do.