Influencing all cultural and financial realms, the American demographic make-up is influenced by four major factors: geography, political history, natural resources giving economic advantages, and immigration patterns:
The New England states and the Eastern coastline, because it their historical connection to Old World politics and economies, form their own demographic, perhaps up to the Mississippi and Ohio Rivers; then the plains in Central North America, where farming flourishes, form a demographic; then the Mountain ranges, Rocky and Sierra; then the western coastline. This is a grossly oversimplified list, but the point is that the physical features of so large a country automatically divide the demographics.
Secondly, the history of the country (for obvious example, the rebel Deep South) divides the demographic -- the cowboy mentality of the near West is another example.
Then, the economics advantages divide a demographic -- farming, cattle ranching, cotton, goldmines, etc.
Finally, the waves of immigration -- Chinese, Polish, Irish, Mexican, etc. -- divide the demographic statistics. Whether dealing with literary influences, financial markets, voting blocks, or entertainment venues, these basic factors must be taken into account in defining or gathering statistics for demographics in the U.S. (Religions, levels of affluence, and many other factors not exclusive to the U.S., are taken into account, also.)