One site where gender is performed and socialized is in sport. Violent sports such as football are fundamental in naturalizing the equation of maleness with violence. Displays of strength and violence, through sports like football, help to naturalize elements of competition and hierarchy as inherently male behaviour. There is considerable evidence that males are hormonally predisposed to higher levels of aggression on average than females, due to the effects of testosterone. However, the violent and competitive nature of sports like football can only be an exclusively masculine domain if girls and women are excluded from participating altogether. The only means through which women are permitted to participate in football is as the passive spectator or cheerleader, although women do sometimes participate in other violent contact sports, such as boxing.