>>213701738 (OP)
Different sports have different corresponding wealth classes.
Things like car racing, horse riding and even snow sports require money, therefore only rich people get early experiences to start training in childhood. So you should expect, and can find, "rich sports" being dominated by rich people.
Ball sports are very cheap and so both rich and poor can start practicing in childhood. So you would expect an even balance of rich and poor, but what you can see is "poor sports" are dominated by poor people. Why is it not even?
I think it's opportunity cost.
A rich or middle class young person has many opportunities to make a career and living. If they choose to become a soccer player they have to dedicate their life to competing against a huge number potential players.
They weigh up this potential career against being a banker, teacher, officer worker etc where they'll be competing against only fellow middle class or rich people.
So the first selection bias is that a middle class person who is 80th percentile sports player and 80th percentile student will probably just become an office worker. Whilst a poor person who is 80th percentile at sport and 20th percentile student will choose sport.
The second selection bias is when a middle class or higher person does choose sport but are in the process of developing their skills when they're interrupted eg. from injury or lack of money. They are then more likely to go for a backup non-sport job than a poor person because those opportunities are still present.
tl;dr
Serbians are poorer than African Americans
