The North was largely settled by English farmers, the inland South by Scots-Irish herders. Anthropologists have long noted that societies that herd livestock in rugged terrain tend to develop a "culture of honor." Since their wealth has feet and can be stolen in an eye blink, they are forced to deter rustlers by cultivating a hair-trigger for violent retaliation against any trespass or insult that probes their resolve. Farmers can afford to be less belligerent because it is harder to steal their land out from under them, particularly in territories within the reach of law enforcement.

As the settlers moved westward, they took their respective cultures with them. The psychologist Richard Nisbett has shown that Southerners today continue to manifest a culture of honor which legitimizes violent retaliation. It can be seen in their laws (like capital punishment and a stand-your-ground right to self-defense), in their customs (like paddling children in schools and volunteering for military service), even in their physiological reactions to trivial insults.

Robert Krulwich has a theory about the blue parts of red states – it involves plankton.