Ethnic groups persist through the boundary that members maintain rather than through the cultural content the boundary encloses, so groups can exchange almost everything and remain distinct.
Barth's 1969 introduction to Ethnic Groups and Boundaries inverted the usual assumption. The question is not what cultural stuff a group has, since that changes and is often shared with neighbours, but how the group keeps producing the distinction between member and non-member. The boundary is the durable object; the culture inside it is not.
In practice
- Two firms after a merger share every process, tool and office, and everyone still knows which company each colleague came from.
- A school's houses have no difference in curriculum, uniform or building, and the rivalry is thirty years old.