My coconut theory

In a tropical island, every one sleeps under a coconut tree. He is waken up by a fallen coconut on his head once a while. He eats the coconut and goes back to sleep. He is lazy due to the nice weather (no need to find shelter) and the nice resource (the coconut tree). He is happy and rich by his own standard.

US is richest by the developed citizens, hard-working immigrants and the huge natural resource per capita . However, Norway is the richest for that size group (3 M) and some smaller oil countries like Brunei is richer.

Norway is rich due to the rich off-shore oil field and the fish export (second in the world after China but far, far #1 per capita wise). It is smart not to boast except in the Olympics to prevent jealousy and invasion.

Singapore is rich due to the port location and hard-working citizens (most are Chinese). When the hard-working folks land to a land of coconuts (i.e. resources), they are natural to be rich as indicated in my coconut theory.