[afterburner]

Yet the culture of those "large family" groups will probably change if they start to dominate, putting less emphasis on large families. The societal impetus to having a large family could come from an insecurity about your cultural block (not the individual impetus mind you, which can sometimes be more important, eg. kids to take care of you in old age or extra kids to overcome infant death). That clan insecurity starts to disappear when you start to become the majority in whatever context you feel is appropriate to consider.