Polyethnicity, also known as pluri-ethnicity or multi-ethnicity, refers to specific cultural phenomena that are characterized by social proximity and mutual interaction of people from different ethnic backgrounds, within a country or other specific geographic region. Same terms may also relate to the ability and willingness of individuals to identify themselves with multiple ethnicities. It occurs when multiple ethnicities inhabit a given area, specifically through means of immigration, intermarriage, trade, conquest and post-war land-divisions. This has had many political and social implications on countries and regions. Many, if not all, countries have some degree of polyethnicity, with countries like Nigeria and Canada having high levels and countries like Japan and Poland having very low levels (and more specifically, a sense of homogeneity). The amount of polyethnicity prevalent in some Western countries has spurred some arguments against it, which include a belief that it leads to the weakening of each society's strengths, and also a belief that political-ethnic issues in countries with polyethnic populations are better handled with different laws for certain ethnicities. In 1985, Canadian historian William H. McNeill gave a series of three lectures on polyethnicity in ancient and modern cultures at the University of Toronto. The main thesis throughout his lectures was the argument that it has been the cultural norm for societies to be composed of different ethnic groups. McNeill argued that the ideal of homogeneous societies may have grown between 1750 and 1920 in Western Europe because of the growth in the belief in a single nationalistic base for the political organization of society. McNeill believed that during World War I, the desire for homogeneous nations began to weaken. Polyethnicity divides nations, complicating the politics as local and national governments attempt to satisfy all ethnic groups. Many politicians in countries attempt to find the balance between ethnic identities within their country and the identity of the nation as a whole.