Upward mobility is lower in US then in other western countries. The feudalism would have much lower mobility, but none of those countries have feudalism and did not had it for years.
Upward mobility is higher in Denmark, where I'm from, so that's what I'm familiar with. I don't think this metric necessarily measures exactly what people expect: The spread of possible outcomes is simply much tighter. Making $65,000/year puts you in the top 10% of earners, while the bottom 10% ends at $19,600/year. When tripling your salary is enough to travel almost the entire scale, obviously more people will do it. The same numbers for the US are $150,000 and $6,560 (not adjusted for purchasing parity or tax, so grain of salt). Or put differently, the 10%-90% income range for Denmark fits inside the 25%-75% income range for the US.
We have a lot fewer poor people in Denmark, and that's undoubtedly a very good thing. But there are also very, very few people who would be considered more than comfortably middle class in the US.
I don't know a good way to measure this, but I think that you can move further, faster upwards in absolute numbers in the US rather than in Denmark. I would expect that it's comparatively easier to for someone born around the 20%-mark to make it to 80% in the US than for someone to make it from 10% to 90% in Denmark, even though about the same journey in absolute numbers.
If I recall right, the mobility was measured by percentage of people who move out of their bracket. E.g. US kid born into 20% bracket is more likely to stay in that bracket. If you move, you make bigger jump (cause differences are bigger), but you are unlikely to move.
