Migrationism
It was a popular explanation for changes in artefact styles amongst processualist archaeologists during the 1960s and 1970s when earlier invasionist approaches were being discredited. Invasion hypotheses became increasingly seen as being too difficult to apply generally as they suggested a permanently warlike prehistory, a view which archaeological finds were beginning to disprove.
Migrationism was seen as a more reasonable explanation, involving long term, more gradual folk movements over distance. It is generally seen as being a slow process, involving family groups moving into new areas and settling amongst the native population. This would provide an opportunity for the transfer of ideas with the new arrivals interbreeding with the pre-existing inhabitants. Going further, the eventual growth or dominance of the immigrant population can explain evidence of social tension.
Other models of explaining social change revolve around 'packages' of ideas travelling over distances through social and economic links rather than people themselves moving. Any of the three explanations can be valid however, depending on the context.
