I've even heard some Italians argue once that 'Italian' should be a panethnicity instead of an ethnicity. It's said that there are large cultural and genetic difference not only between northerners and southerners but even between Neapolitans, Calabrians, Sicilians and Apulians.
But I'm not an expert on that.
IMO Italians aren't an ethnicity but a nation formed in 19th century on linguistic and geographic basis. The people who would through time start to identify themselves with Italian national identity were ethnically and culturally diverse, part of different cultural spheres and different historic states on the peninsula. Particularly in the south (what was "The Kingdom of Two Sicilies" before the unification) the Italian-speaking people had a strong regional identity of their own and many of them opposed the new Italian nation-state (economic and religious/ideological reasons also played a role).
Italian national identity then consilidated through 20th century. That's another aspect we have to consider when comparing Italians from Italy and Italian-Americans. Majority of Italian-Americans immigrated to USA from 1890s to 1920s and were not part of major historic events that in Italy considerably shaped Italian national identity - WWI and the "patriotic experience" of fighting for Italian homeland, the rise of Fascism and Fascist rule (very important part of Italian history, for a number of reasons), WWII, the loss of Istria and Dalmatia, the end of monarchy, the cold war in Europe, post WWII decades and political struggle between Christian Democracy and Communist Party (PCI), economic miracle in 50s and 60s, socio-political turmoil in "the Years of Lead", the fact that Italy has now for decades been (de facto) a social-democratic state (like the rest of Europe) and in this regard different from the USA etc... These things shaped Italian identity, mentality and political culture (and culture in general) but had no effect on the Italians in the USA. I doubt Italian-American population in general knows much about these events. The big majority of them probably didn't care what went on in Italy because it didn't concern them. Instead, they were strongly influenced by the American environment and by what was going on in the USA, which on the other hand didn't concern Italians in Italy. It's the same in any other community of economic immigrants.
