Even if they immigrate as refugees on rusty boats, or lose everything in a holocaust or internment camps. Or, even if they win the lottery. After three generations, they hit their level, whether it's at the bottom or the top. And then they stay there.
It's IQ, an absence of stimulation-seeking behavior, a long mental time horizon, and a non-susceptibility to addictive chemicals or behaviors, and an absence of costly mental and physical diseases. It's genetic.