Violence in sports usually refers to violent and often unnecessarily harmful intentional physical acts committed during, or motivated by, a sports game, often in relation to contact sports such as American football, ice hockey, rugby football, lacrosse, association football, boxing, mixed martial arts, wrestling, and water polo and, when referring to the players themselves, often involving excessively violent or potentially illegal physical contact beyond the normal levels of contact expected while playing the sport. These acts of violence can include intentional attempts to injure a player or coach by another player or coach, but can also include threats of physical harm or actual physical harm sustained by players or coaches by fans or those engaging in the spectating of sports, or threats and acts of violence performed by fans or spectators upon opposing fans or other spectators. Causes There are two major theories on the cause of violence in sports. One theory holds that humans have an instinct for violence, developed during a time when early human ancestors had to resort to violence and aggressiveness to survive and reproduce. Another theory deals with the sociological aspects of violence in sports, stating that sports are "mock battles" which can become actual battles due to their competitive nature. Violence by athletes Through a "civilizing process", many modern sports have become less tolerant of bloodshed than past versions, although many violent aspects of these sports still remain. Athletes sometimes resort to violence, in hopes of injuring and intimidating opponents. Such incidents may be part of a strategy developed by coaches or players. In boxing, unruly or extremely violent behavior by one of the contestants often results in the fighter breaking the rules being penalized with a points reduction, or, in extreme cases, disqualification. Outlawed tactics in boxing include hitting the opponent on the back of the head, under the belly during clinching, and to the back. Other tactics that are outlawed, but less seen, are pushing an opponent extremely hard to the floor, kicking, or hitting repeatedly after the round has ended. Similar actions have also happened in ice hockey and Australian Football League matches. Ritual violence High school, college, and even professional sports teams often include initiation ceremonies (known as hazing in the USA) as a rite of passage. A 1999 study by Alfred University and the NCAA found that approximately four out of five college US athletes (250,000 per year) experienced