If we define the Bible Belt as a sequence of Mid-Western/Southern states with a high degree of Protestant fundamentalism, Stark & Bainbrisge claim it doesn't actually exist.
One reason might be that questions used to ascertain beliefs, such as "Would you say that you have been 'born again', or have had a 'born again' experience - that is a turning point in your life when you comitted yourself to Christ?" The media will comment on the concentration of "yes" answers in the 'Bible Belt', but notice that the question itself uses distinctly evangelical Protestant vocabulary. Rather than exposing religiousness, it merely uncovers Protestantism.
While the 'Bible Belt' does have a concentration of Protestants, their degree of fundamentalism - as measured by church membership, church attendance, conventional religious belief, prayer, or religious experience - is not significantly different from anywhere else in the USA.
So the 'Bible Belt' is not a highly religious area, although it is religiously homogenous.
A crucial constitute of the 'Bible Belt' states is that they're agricultural, creating rural domination of the state legislature. A lack of diverse religions breeds intolerance, and that can therefore be enacted into law.
This is a very nice example of how economists' expect society to act. On the surface, conventional wisdom tells us that the 'Bible Belt' is a mystical region of high fundamentalism. The sociologists comeback might well be to blame a northernly-bias media, wanting to dismiss the south as ignorant, backwater, bible-bashing red necks. The economist will assume that preferences are fairly stable across people and over time, and that observed differences (for example the debate over evolution appearing in textbooks) is down to some institutional perversion.
The people of the south are no more religious than anywhere else. But a high degree of homogeneity, coupled with the constitutional ability for a minority to control the lawmakers, creates an illusion that the media have misunderstood.