Ethnic violence is a form of political violence which is expressly motivated by ethnic hatred and ethnic conflict. Forms of ethnic violence which can be argued to have the characteristics of terrorism may be known as ethnic terrorism or ethnically motivated terrorism. "Racist terrorism" is a form of ethnic violence which is dominated by overt racism and xenophobic reactionism. Ethnic violence which is perpetrated in an organized, sustained form is known as ethnic conflict or ethnic warfare (race war), in contrast to class conflict, where the dividing line is social class rather than ethnic background. Care must be taken to distinguish ethnic violence, which is violence which is motivated by an ethnic division, from violence that is motivated by other factors and just happens to break out between members of different ethnic groups (political or ideological). Violent ethnic rivalry is the subject matter of Jewish sociologist Ludwig Gumplowicz's Der Rassenkampf ("Struggle of the Races", 1909); and more recently, it is the subject matter of Amy Chua's notable study, World On Fire: How Exporting Free Market Democracy Breeds Ethnic Hatred and Global Instability. Some academicians would classify all "nationalist-based violence" as ethnic violence, a classification which would include the World Wars and all of the major conflicts between industrialised nations which occurred during the 19th century. Causality and characteristics There are various potential causes of ethnic violence. Research which has been conducted by the New England Complex Systems Institute (NESCI) has shown that violence results when ethnic groups are partially mixed: neither clearly separated enough to reduce contact nor thoroughly mixed enough to build common bonds. According to Dr. May Lim, a researcher who is affiliated with NECSI, "Violence normally occurs when a group is large enough to impose cultural norms on public spaces, but not large enough to prevent those norms from being broken. Usually this occurs in places where boundaries between ethnic or cultural groups are unclear." This theory also states that the minimum requirement for ethnic tensions to result in ethnic violence on a systemic level is a heterogeneous society and the lack of a power to prevent them from fighting. In the ethnic conflicts that erupted after the end of the Cold War, this lack of outer controls is seen as the cause; Since there was no longer a strong centralized power (in the form of the USSR) to control the various ethnic groups, they then had