I think all of these analyses contain a critical flaw in that they ignore the effect of evolution. Any characteristic that increases reproduction rates that is passed down to the next generation, whether that characteristic is cultural, location-specific, or genetic, will eventually become the majority. (And yes, cultural beliefs and location can be "passed down" to some degree.) This will result in eventual birth increases as pro-fertility adaptations build up. You can't assume that the system is static and that trends will continue when the math of evolution inherently promotes a pro-reproduction bias.