ישובו של עולם

 In the Talmud we find two main ways of being עוסק בישובו של עולם loosely translated as living a productive lifestyle. One way of being עוסק בישובו של עולם is the obvious one which is actually having a job which delivers goods and/or goods and services to mankind. Or starting a business that accomplishes the same. The other one is having children. The Talmud in a number of places quotes the Pasuk in Isaiah 45:18 לֹא תֹהוּ בְרָאָהּ לָשֶׁבֶת יְצָרָהּ The world was created to be inhabited and explains that Pasuk to mean that "The world was created for procreation"

Generally healthy people have an instinctive desire to live productive lifestyles. The concept of being עוסק בישובו של עולם has been firmly planted in humankind's natural organic psyche. The more a person is עוסק בישובו של עולם the more he feels fulfilled. 

This might explain an interesting phenomenon. We find that people in underdeveloped societies or people in developed societies but themselves less developed generally speaking have much higher birth rates than people that are more advanced or successful. There is almost a direct inverse correlation (barring external factors) between the wealth of a society and the birth rate. This inverse relationship exists when measuring comparing continent to continent, country to country, city to city, or neighborhood to neighborhood. 

The explanation is simply that the innate drive to be עוסק בישובו של עולם is satisfied by a person if he feels accomplished in his daily life. To the extent that he doesn't feel as accomplished in his delivering goods and services to society he has to fill that void somehow. That void is filled by procreating which is another way of being עוסק בישובו של עולם (maybe because those offspring will produce for others what he didn't).