An argument can be made for defining "people group" according to the highest of the two barriers (understandability and acceptance), in some circumstances. If understandability is the most important barrier, then a linguistic, or an ethno-linguistic, approach is used. With this approach, one people group doesn't speak more than one language (apart from occasional bi-lingual individuals), although more than one people group may speak a given language if cultural or dialect differences warrant. In most of the world, this is the approach Joshua Project uses.
The Karakalpak in Uzbekistan are an example of a people group defined by understanding.
If the cultural / relationship barrier is the greatest barrier (as it often is in South Asia), then we treat caste / tribe as the first criterion for separation. With this approach, one people group may speak more than one language. And as with the first approach, one language may be spoken by more than one people group. (Note that multi-lingualism is not so much in view as the situation where some individuals in the group speak language A and others speak language B. Some may be multi-lingual, but that's not the main issue.)
The Sonar in India are an example of a people group defined by acceptance.
Allowing one people group to speak more than one language is a compromise. It is a way to present a somewhat simplified picture of the church planting task, at the risk of over-simplifying the understandability barriers within people groups.
Joshua Project's first purpose is to support church planting and discipleship efforts, and we define "people group" with that goal in mind.
Used by permission from Joshua Project - https://joshuaproject.net/resources/
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