Ethnic cleansing occurs as members of one ethnic group employ tactics designed to purge all members of another ethnic group from a particular location. One example of this policy occurred in the early history of the United States of America. Native Americans were resettled and made to relocate to predetermined reservations numerous times. The Trail of Tears is perhaps the most well known North American instance of ethnic cleansing. Such cases are now prohibited by internationally signed treaties and agreements. Any attempt to eradicate a portion of a particular ethic group in any region is now considered genocide. Perhaps the most infamous example of an ethnic cleansing occurred in the midst of World War II, as the Nazis systematically executed Jews and members of other groups they considered racially inferior. This was neither the first nor the last time that people would commit atrocities of this nature under the guide of purifying the human genome.
Ethnic cleansing takes its theoretical roots primarily in Social Darwinism and eugenics. The eugenics movement propagated from the ethnocentric notion that certain races were inferior to others from a biological perspective. This train of thought was particularly prominent through the hereditarian train of thought that was popular in the early twentieth century. These theories were not only supported by new emerging evidence at the time (natural selection and genetics), but they were hard to refute in the absence of more modern theories and observations. The idea that racial phylogeny could cause an individual to have predetermined dispositions, intelligence and cognition was highly popular during this time period. Cesar Lombroso, an Italian criminologist, attributed the majority of crime to his concept of atavism. The atavistic man was a genetic throwback, bearing numerous similarities to the great apes and earlier Hominid species. It is now understood that racial phylogeny plays nowhere near as large a role in determining personality, affect and cognition that previously thought and more weight is placed upon environmental factors and external stimuli.