As Italian I could try to give you an explanation about the difference between North and South Italy. Basically it derives from the unification process, during the last part of 19th century. Someone thinks the South payed more in this process. Someone else argues that it depends on economic and political differences present before the unification. North was an ensemble of little states, South a large state related to Spanish monarchy.

Mind-boggling amounts of corruption (for a western country) as well as various mob related issues that are concentrated in the south. Historically, Italy has had (and arguably still does) poor transport infrastructure, so being in the northern part of the country is closer to the rest of Europe.

I have a very simple explanation - geography and climate. The north has cold winters, if one does not work hard they will freeze and starve. Hard work is embedded in the region for generations. In the south you won't freeze in winter, so you don't need to prepare fuel in summer, life has been slower and does not demand as much of hard work to survive. The summer heat in the south also inhibits the activity somewhat, and north may have a tad easier access to trade... I believe these two factors - geography and climate - have a prevalent influence on the development of all societies anywhere in the world.

North/South divide predates ww2.