Population is a major driver.  What are the fundamentals behind it? Sex as a need and desire is obvious, but the desire to have a family might be more complicated, like having a place in the great chain of being, of fitting in to a community. Or the  way identity replicates the values of the community. In particular the need for security – should it be met by personal family, or by a larger stable community?

But these are all psychologically complex. Place in community might come from art or service. So I would have to conclude that population drive is based on the way the community interprets life, and if family is a major or minor part of that.  I once spent a month with a friend on Bali and we got to know a young man who was terrified that, not being married, there was no one to overlook his funeral and the release of his soul back to the gods.