Sportingintelligence uses a calculated metric they call "average first-team pay," which is meant to make up for the number of players on a first string team for the different sports. Soccer, for example, could have 25 players considered while for basketball, the pay for 13 or 15 players was considered.
Using a statistical measure called the Gini coefficient, CNBC looked at the level of equality between teams in each of the 17 leagues. The Gini coefficient, which is often used to describe levels of income inequality between nations, is a figure between 0 and 1. If a nation's Gini coefficient is 0, it means that all citizens have the same income, while if it's 1, it means the nation's income is entirely in the hands of a select few citizens. The 28 nations that make up the European Union have a Gini coefficient of 0.3, according to eurostat, far below the U.S.'s 0.41.
But the opposite is true when it comes to sports.
It turns out that European sports leagues tend to be less equal than American leagues in their distribution of average annual pay. The Scottish Premier League—with its Celtic team making six times any other team—has the highest Gini coefficient, at 0.53. Spain's La Liga is also above 0.5, with Barcelona and Real Madrid ranking high as outliers.
Stateside, it's the all-American NFL that has the most equal distribution of pay. The NFL's Gini coefficient is 0.04, ranking it 14th among the leagues. Sportingintelligence doesn't include endorsement deals or any other outside pay in its report.
Read More Why does Tom Brady make so little in endorsements?
Major League Soccer has a Gini coefficient of 0.31, the highest among American sports. That's interesting because like the NFL, MLS has a salary cap that should—theoretically—keep salaries within the same ballpark. But each MLS team has "designated" players whose salaries aren't counted towards the cap, which adds to differences within the league. "The salary structure of the MLS seems to offer a perfect recipe for inequality," two researchers at the Harvard Sports Analysis Collective recently wrote.