In practice, attention is credibility. You can look at this as a cultural problem[1], a psychological finding[2], a propaganda tool[3], a business model[4], etc.

[1] http://en.wikipedia.org/wiki/Neil_Postman [2] http://en.wikipedia.org/wiki/Availability_heuristic [3] http://en.wikipedia.org/wiki/Public_Opinion_(book) [4] http://en.wikipedia.org/wiki/Jonah_Peretti

I guess credibility is a poor word choice. I meant something more like trustworthiness (which might tend to build credibility with a certain demographic...).