I'd say impulsivity and asymmetric extra opportunity. A European can usefully learn English before knowing what they'll use it for, pounce on whatever opportun

I'd say impulsivity and asymmetric extra opportunity. A European can usefully learn English before knowing what they'll use it for, pounce on whatever opportunity arises in the more impulsive US economy, possibly multiple times in different places, make some extra money for some years, and then eventually return home to a lower cost of living. Going the other way requires more planning and more specific goals for no bigger a payoff than can be found domestically, which naturally filters out many people.