Economists call this idea the uncertainty of outcome hypothesis (UOH), and it has been the subject of extensive research in behavioral economics.
[3] As of the 2020s, empirical evidence increasingly pointed to the conclusion that promotion and relegation, standing alone, was insufficient to ensure adequate parity in any given game.
The appearance of parity in the NFL over the course of the entire season may be something of an illusion; the New England Patriots, in particular, were noted for a prolonged dynasty in the 21st century under starting quarterback Tom Brady and head coach Bill Belichick, which league policies failed to break up, while their division rivals, the Buffalo Bills, simultaneously suffered through a 17-season playoff drought but may also have been harmed by Buffalo's small market, high taxes, and poor reputation as a destination city.
A league with parity would, in theory, allow such struggling teams to identify and fix those issues and ensure that a dynasty cannot take hold.
[5] Two notable exceptions were the Baltimore Stallions, which reached the Grey Cup in both their seasons in the Canadian Football League and won in their latter appearance, and the Vegas Golden Knights, which reached the Stanley Cup Finals in their inaugural season in the National Hockey League.
The All-America Football Conference collapsed in part because one of its teams, the Cleveland Browns, dominated the league throughout its four-year existence.