Migrationism and diffusionism
In the history of archaeological theory the term migrationism was opposed to the term diffusionism (or "immobilism") as a means of distinguishing two approaches to explaining the spread of prehistoric archaeological cultures and innovations in artefact. Migrationism explains cultural change in terms of human migration, while diffusionism relies on explanations based on trans-cultural diffusion of ideas rather than populations ("pots, not people"). Western archaeology the first half of the 20th century relied on the assumption of migration and invasion as driving cultural change. This was criticized by the processualists in the 1960s and 1970s, leading to  a new mainstream which rejected "migrationism" as outdated. Since the 1990s, there has been renewed interest in "migrationist" scenarios, as archaeologists attempted the archaeological reflexes of migrations known to have occurred historically. Since the 2000s, the developments in archaeogenetics have opened a new avenue for investigation, based on the analysis of ancient DNA. Kristiansen (1989) argued that the reasons for embracing "immobilism" during the Cold War era was ideological, derived from an emphasis on political solutions displacing military action. History. "Diffusionism" in its original use in the 19th and early 20th century did not preclude migration or invasion. It was  rather the term for assumption of "any" spread of cultural innovation, including by migration or invasion, as opposed "evolutionism", assuming the independent appearance of cultural innovation in a process of parallel evolution, termed "cultural evolutionism". Opposition to migrationism as argued in the 1970s had an ideological component of anti-nationalism derived from Marxist archaeology, going back to V. Gordon Childe. Childe in the interwar period combined "evolutionism" and "diffusionism" in arguing an intermediate position that each society developed in its own way, but strongly influenced by the spread of ideas from elsewhere. In contrast to Childe's moderate position, which  did allow the diffusion of ideas and even moderate migration, Soviet archaeology adhered to a form of extreme evolutionism, which explained all cultural change due to the class tensions internal to prehistoric societies. "Migrationism" fell from favour in mainstream western archeology in the 1970s. Adams (1978:483f.) described migrationism an "ad hoc explanation for cultural, linguistic, and racial change in such an extraordinary number of individual cases that to speak of a migrationist school of explanation seems wholly appropriate". Adams (p. 484) argued that the predominance of migrationism "down to the middle of the last [19th] century" could be explained because  it "was and is the only explanation for culture change that can comfortably be reconciled with a literal interpretation of the Old Testament", and as such representing an outdated "creationist" view of prehistory, now to be challenged by "nonscriptural, anticreationist" views. Adams (p. 489) accepts only as "inescapable" migrationist scenarios that concern the first peopling of a region, such the first settlement of the Americas "by means of one or more migrations across the Bering land bridge" and "successive sweeps of Dorset and of Thule peoples across the Canadian Arctic". While Adams criticized the migration of identifiable "peoples" or "tribes  was deconstructed as a creationist" legacy based in biblical literalism, Smith (1966) had made a similar argument deconstructing the idea of "nations" or "tribes" as a "primordalistic" misconception based in modern nationalism. Historian Alex Woolf notes that "in the minds of some scholars, immobilism was charged with a left-wing caché ; those who showed too much interest in the ethnic or racial origin of the people they studied were, it was hinted, guilty of racist tendencies." While mainstream western archaeology maintained moderate scenarios of migrationism in spite of such criticism, it did move away from "invasionism". The mainstream view came  to depict prehistoric cultural change as the result of gradual, limited migration of a small population that would consequently become influential in spreading new ideas but would contribute little to the succeeding culture's biological ancestry. Thus, the mainstream position on the Neolithic Revolution in Europe as developed (notably by German archaeologist Jens Lüning) since the 1980s, posits that "a small group of immigrants inducted the established inhabitants of Central Europe into sowing and milking" in a process spreading "in swift pace, in a spirit of 'peaceful cooperation'" Migration was generally seen as being a slow process, involving family groups  moving into new areas and settling amongst the native population, described as "demic diffusion" or "wave of advance", in which population would be essentially sedentary but expand by the colonisation of new territory by succeeding generations. The question remained intractable until the arrival of archaeogenetics since the 1990s. The new field's rapid development since the 2000s has resulted in an increasing number of studies presenting quantitative estimates on the genetic impact of migrating populations. In several cases, this has led to a revival of the "invasionist" or "mass migration" scenario (in the case of the Neolithic Revolution in Europe ), or at least suggested that the extent of prehistoric migration had been underestimated (e.g. in the context of Indo-European expansion, it was estimated that the people of the Yamnaya culture in Eastern Europe contributed to 73% of the ancestry of individuals pertaining to the Corded Ware culture in Germany, and to about 40–54% to the ancestry of modern Central & Northern Europeans.) In British archaeology, the debate between "migrationism" and "immobilism" has notably played out in reference to the example of the Anglo-Saxon settlement of Britain. The traditional view of the process, broadly supported by the available textual evidence , was that of a mass invasion, in which the Anglo-Saxon incomers drove the native Romano-British inhabitants to the western fringes of the island. In the latter half of the twentieth century, archaeologists pushed back against this view, allowing only for the movement of a small Anglo-Saxon "warrior elite" that gradually acculturated the Romano-Britons. In recent years, however, due to a combination of factors (including genetic studies of British populations and observable migrations in the present day), most scholars in Britain have returned to a more migrationist perspective, while noting that the scale of both the settlement  of the Anglo-Saxons and the survival of the Romano-Britons likely varied regionally.