There are a few of reasons for it.
Part of it goes back to the days when medicine was not very advanced so a higher birthrate meant more would survive and offset the high infant mortality rate.
A farmer who was poor-needed sons to tend the field,so to raise more crops.
Having no Social Security, those sons were also there to support he and his wife in their old age.
Historically churches promote larger families to build up their parrishes.
The poor have less access to birth control information and because of this tend to listen to their religion and superstition, rather than science which generally they find hard to understand because of poor education-and so tend to dicourage birth control.
Also, sex is a free entertainment, as well as a release from the daily grind of eeking a living.
In more affluent countries.
Better medicine, A Social Security type system, More varied income opportunity-and more choices in lifestyle opportunities-all of these result in a lower birthrate, as it just is no longer necessary to produce lots of children, which also are more expensive to raise. Also because of a superior educational system they are less inclined to follow the strict teachings of their religion.
Nature though has away of leveling out. When a species becomes to comfortable to adapt, she throws a monkey wrench into their system. Every culture that reaches the top of the pyramid eventually falls off. In this case, the white race built the largest pyramid ever. The fall will be just as dramatic.
Frank
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"The luxury of not getting involved departed with the last lifeboat Skipper..."