The extractive rent-seeking (slavery being the most extractive economic and political institution possible) South was significantly lagging even before the rebellion, which was crushed by the United States whose other regions had more inclusive economic institutions.
In more recent times the authors compare two towns across the US-Mexico border: Nogales, AZ to to Nogales, Mexico. Even thought the language and ethnicity is mostly the same (95% Hispanic), with same climate and what not - Nogales in the United States has three times the GDP per capita of Nogales, Sonora. The difference is more inclusive economic and political institutions in the United States - while Mexico has suffered under one party rule for decades.