↑ Guttman, Allen. "The Diffusion of Sports and the Problem of Cultural Imperialism". In Eric Dunning, Joseph A. Maguire, Robert E. Pearton. The Sports Process: A Comparative and Developmental Approach. Champaign: Human Kinetics. p. 129. ISBN0880116242. สืบค้นเมื่อ 2008-01-26. the game is complex enough not to be invented independently by many preliterate cultures and yet simple enough to become the world's most popular team sportCS1 maint: Multiple names: editors list (link)
↑ Frederick O. Mueller, Robert C. Cantu, Steven P. Van Camp. "Team Sports". Catastrophic Injuries in High School and College Sports. Champaign: Human Kinetics. p. 57. ISBN0873226747. สืบค้นเมื่อ 2008-01-26. Soccer is the most popular sport in the world and is an industry worth over US$400 billion world wide. 80% of this is generated in Europe, though its popularity is growing in the United States. It has been estimated that there were 22 million soccer players in the world in the early 1980s, and that number is increasing. In the United States soccer is now a major sport at both the high school and college levelsCS1 maint: Multiple names: authors list (link)
