Some beefy guy walking the opposite way bumps into a student as he walks past, then says, "Watch it, asshole," before marching away. When the student comes in to take the math test, the researchers take their blood pressure, check their hormones. And if you're from the American South, your blood pressure will be higher, and you'll be more stressed out. This impacts your judgement and how you respond to a given situation.
This is because, by best evidence, the American South was settled by herders and pastoralists from northern England and Scotland, who had a culture of honor. Centuries later, there's still a residue of that. So this makes culture not such an intangible factor of brain development and behavior. Within minutes of birth, this kind of training starts.