Thanks to Neil Moore for this link. The article quotes one Prof Forrest: "We have identified four sports where there is virtually no chance that anyone from a poor country can win a medal - equestrian, sailing, cycling and swimming"
Meanwhile today the UK just won a gold medal in triathlon which combines two of those sports (plus running.) And celebratory BBC news article says "Further medals could follow on Tuesday in cycling, dressage and windsurfing." Which are - you guessed it - three of the four rich-country sports. With the triathlon gold partly for swimming to complete the set.
I am genuinely happy about British success but at the same time very saddened that poor countries are eliminated from competition in many key areas of the games. Not the way the world's greatest sporting event should be.
Athletics is more or less an exception. If you can run fast you can make enough money to compete at the highest level wherever you are. And critically be identified early enough to be trained well enough in the USA. So I think it's wonderful that the 10000m champion was born in Somalia, grew up in the UK and ran for Britain, and lives in Portland Oregon.