What are the causes of the western worlds declining birth rate?

Fewer and fewer westerners are having kids and even kids at a later date in life. Not many families have 2 kids or more. So the government has this idea that bringing in people from 3rd world countries with 10 + kids is a method to solve the countries declining birth rates.
Yet western born citizens don't seem to be having kids like those people in impoverished countries.
What are the causes of the western worlds declining birth rate?
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