For many of the billion spectators who watched the football World Cup opening in Rustenburg, South Africa, the word "globalization" may have come to mind. From advertisers to spectators, football embodies globalization like no other sport. And for players, football embodies globalization like no other profession.

The market for professional football players is, by far, the most globalized labor market. A Nigerian or Brazilian football player can get a job more easily in Europe or Japan than a skilled surgeon or engineer. Out of some 2,600 professional players in the five top European leagues – England, Spain, Italy, Germany and France – almost 800 are expatriates, defined as those born and recruited in a county different from the one where they play, according to data published by Professional Football Players Observatory for the last football season.

The greatest push to the free movement of labor in football came in 1995 after the so-called Bosman ruling. Belgian player Jean-Marc Bosman complained to the European Court of Justice against rules that then limited the number of foreign players to two or three per club. The rules were, Bosman argued and won, in flagrant violation of freedom of movement and non-discrimination labor laws within the European Union. The ruling lifted limits on EU players, and soon other limits on African, East European or Latin American players were formally abandoned or made irrelevant. Thus, global mobility in one small market, for top professional football players, became almost complete. Today, many of the best clubs have no players at all from their "own" countries. The Inter Milan squad had no Italian starters only a few weeks ago when winning Europe's most prestigious competition, the Champions League.

Suppose that similar global mobility of labor were to spill to other professions? If medical doctors could move with equal ease from Cameroon to Spain or Italy, as Samuel Etoo, the Inter Milan striker did; or engineers could move from Ivory Coast to France and then England, as London Chelsea's Didier Drogba did?

Football could provide clues to what this new world of mobility, largely unhindered by national borders, might look like. Globalization of the world's most popular game is responsible for two developments:

The first one cannot be easily quantified, but most observers agree that the quality of the game has improved: players have greater physical stamina, with better ball control and technique.

But also, global mobility of labor combined with a capitalist system, in which the richest clubs can buy the best players without salary caps or other limits, concentrates quality more than ever before. A handful of the richest football teams buy the best players and collect the most trophies, thus boosting their popularity, developing an international fan base, selling more jerseys and advertisements, adding to their coffers and, in turn, buying better players.

The gap between the top clubs and the rest has widened in key European leagues. During the last 15 years, all English football championships but one were won by the so-called "Big Four": Manchester United, Chelsea, Arsenal and Liverpool. The concentration is greater in Italy: Only once during the last 20 years has a non top-four club won the Italian Serie A. It's no surprise that the top four Italian clubs, like the top four English clubs, are on the list of the 20 richest clubs in the world. In Spain, Real Madrid and Barcelona shared 17 of the last 20 championships. In Germany, 13 of the last 16 championships were won by two clubs.

Winners of the European Champions League are consistently from a narrowing circle of top, richest clubs. The Champions league is played annually, and over a five-year period, there theoretically could be 40 different teams in the quarter-finals. In the mid-1970s, that number was around 30. Since then, every successive five-year period produced a smaller number of teams, with only 21 in the period ending in 2010. The day could come when the same eight teams play in the quarterfinals, year in and year out – a trifle boring indeed.

At the club level, globalization combined with commercialization thus produces two outcomes: better quality of the game, which is tantamount, in economics, to greater output; and greater concentration of winning clubs, which is tantamount to greater inequality.

The question is, can greater output be preserved while mitigating the effects of inequality? Yes, though not at the club level discussed so far. Only at the national level – say, team USA, team England – where different rules imposed by the Federation Internationale de Football Associations (FIFA) apply. At the national level, expatriates cannot play for the countries where they live, but must play for countries of origins. To some extent, this reverses the "leg drain," most famously so once every four years during the World Cup – akin to Cameroonian doctors based in France returning from time to time to perform operations in Douala or Yaoundé. For example, in this World Cup, out of 23 footballers on either Cameroonian or Ivorian roster, one plays at home. For Ghana, there are three domestic players (out of 23), and in the case of Nigeria – zero. Even reclusive North Korea has on its roster three players who do not play in the country.

It would seem that this reversal should equalize the outcomes somewhat, particularly as players from small African leagues play in larger English or Spanish leagues, much as a doctor from the developing world returns with skills and connections acquired after an education at Stanford or Yale.

And indeed, differences between national teams, most notably at World Cup games, have steadily decreased. At the last three World Cups the average difference in goals per game between winning and losing teams has ranged between 1.2 and 1.3, as opposed to 1.7 some 30 years ago. The decrease is sharper for the top eight national teams: from 1.6 to 1.

In other words, the gap in performance between national teams is small, with most games ending with one goal separating the two sides, in ties, or overtime tie-breakers in the knock-out stage. Unlike at the club level, the top-eight tier among nations competing for the World Cup is more open: Since 1986, at least one "new" national team, which has never before been member of the elite eight, has made it to this august stage: Ukraine in 2006; Senegal, Turkey, South Korea in 2002; Croatia and Denmark in 1998; Romania and Bulgaria in 1994, etc.

Nation-level football suggests that globalization can be made sustainable and reduction in inequality is surely part of that sustainability. But for that, global rules must accompany globalization, whereby losers get something for agreeing to play the game. Translated into everyday language of economics, some brain drain may be reversed by imposing special short-term return duties on migrants.

This would require international coordination whereby rich countries would issue work permits, obliging the migrants to spend one of, say, each five years working in their own country. A stint back home would be a condition for extension of work permit, with the system continuing for three or so rounds. To have a real bite, the system would require policy coordination by most rich countries.

Whatever its exact modalities, a more open-minded immigration policy should be combined with special duties for migrants, the biggest beneficiaries from freer movement of labor, from which their countries of origin would derive some benefit too.

Branko Milanovic is a professor at the School of Policy, University of Maryland. His books include "Worlds Apart: Measuring International and Global Inequality" and "Income and Influence: Social Policy in Emerging Market Economies." This reprinted by permission of theYale Center for the Study of Globalization