system
A chat on subreddit /r/NoStupidQuestions
human
Lots of social reasons, but one of the economic reasons is a difference in the relative cost. 

In under developed countries, kids are an asset which gives a family unit more manpower, security, and a way to care for the elders. 

In developed countries, kids are a liability that make it difficult to work a full time job and they incur costs for their entire life.
gpt
&gt;Lots of social reasons

Including:

* Better education
* Better access to birth control
* Better sex education of youth
* Closer to equal rights for women
* More job opportunities for women
* Better healthcare (so higher survival rate per child)

all of which correlate with lower birth rates.

**TL/DR: uneducated poor teens have lots of unprotected sex**