[TeMPOraL]

From what I heard, in very poor countries children are often treated as investment. A child can start paying itself off by working as soon as even 5 years after being born. As a parent, you want someone to take care of you when you're old, and given high children mortality, you're better off making more of them in hope at least one survives to adulthood. Basically, those people are too poor to afford not having many children.

[JoeAltmaier]

This is rapidly becoming ancient history. There's a wonderful youtube video showing all the countries on earth, moving on a multimensional chart by infant mortality and standard of living. The origin is ideal. All countries but 1 or two have shot like arrows toward the origin over the last century.

[exch]

This talk by Hans Rosling may be what you are referring to.

https://www.youtube.com/watch?v=dymPP9RhPjw