Tennis, I don't know. Boxing is easy. The decline has been building for a long time. The disappearance of the white ethnic slum reduced the number of potential American boxers by a large number. You just can't come from Mt. Lebanon and be a top flight boxer. Guys like Paul Spadafora used to be a dime a dozen in most big cities. Not the case anymore. Do you think Cranberry could produce a boxer like Fritzie Zivic, or a hard *** like Joey Diven? That process has been underway for 70 years though. Black boxers were able to fill the gap for most of those 70 years. Significant numbers of blacks still live in rough areas, so it's not hard to find tough guys, and hard *****. So why aren't they boxing? Well, the mid sized guys still are to a degree. Black American champions can still be found from welterweight, up to super middleweight. The change is in the high weight classes. Why? Because those guys almost to a man choose football when they reach the age where kids used to first put on the gloves. The reasons are obvious. Despite the worries about concussions in football, the risks are far below that of boxing, which is basically the concussion game. If you had a 14 year old son who was already 6 feet tall, and 190 pounds, would you want him to box, when he could play football instead? Football today pays huge money, and while the odds are still long, it's much easier to reach the NFL, than to become a world class boxer. While there are probably guys in the NFL who could have been good boxers, there are likely many more who never played beyond college, or even high school who could have boxed, but chose football at the age when they would have needed to be learning the fight game. It's hard to really become a good boxer when you don't put the gloves on until you're 20-21. Smaller guys are still around in boxing, because many were just too small to have a realistic chance to do anything in football. You might notice that more than a few of the handful of US boxers in the high weights are guys who have been in jail, and didn't have a chance to play football. Mike Tyson is maybe the best example of this. Mike took up boxing while in reform school. Do you think Mike would have boxed if he had grown up in Aliquippa, or Duquesne? I doubt it. He would have likely been a hard hitting linebacker, maybe good enough to play for some college. Kids in the other countries are still growing up the hard way, like American kids did before WW2. That's why they're dominating boxing today.