I grew up in greater NYC in an area with many immigrants, and I am myself the grandson of Italian immigrants. A teacher in high school jokingly challenged us to sit on our hands while we talked. Without thinking about it, I found myself lifting my butt to free my hand in order to make a conversational point and then sticking my hand back under. Repeatedly.

Much laughter ensued.

Anyway, it seems that the habit transferred to the new world without anyone really trying.