By the 1970s, Anglo-American archaeology sought the motivations that were likely to have brought about material change, but had operated internally within the systems of human behaviour. It was assumed that various elements of the behavioural systems represented patterns of social behaviour. One consequence was the realisation that any social system was unlikely to have operated in isolation (for example foreign and exotic materials were often modelled as playing a key role internally to a social system). World Systems Theory therefore impacted upon the way the New Archaeology began to adopt social theory.