Population boom

[The Valkyrie]

Well, yeah - population grows when the birth rate > death rate. That's kind of tautological.

Number of offspring isn't directly a factor, not as much as you might think. Every couple could have 15 kids, and if only three reached reproductive age, your population growth would be slower than if every couple had four that all reached reproductive age. All else being equal, of course.

It seems to me that mortality is the more important factor, all else being equal.

but if they had only one child!?

Look at China.