[nobodysbusiness]

I think your math is off.

Imagine that you have a population of 200 humans (100 male, 100 female). Only 40% of the women want kids, which, in this case, means that 40 women want kids. They each have three kids, for a total of 120 kids produced by the population of 200 adults. I don't think that this would lead to population growth.

[w0utert]

You're right, unless you would somehow ensure that of the 3 kids every mother has, 2.5 are female ;-)

Anyway, looking at the worlds' population right now, I don't think we have to be afraid mankind will go extinct anytime soon because not enough children are born.