Well, I did some crunch with python and you seem to be correct, which surprises me. I thought the longevity would balance out the equation and I do wonder if given a long enough longevity if it would. Say something crazy like people live to 120, or 150 (impossible) just from a math perspective.

Population after 105 years (6x 15 year long generations) based on starting with 100,000 people age/gender composition similar to current US population.

1.1: 34,627

1.5: 42,586

2.0: 50,546

2.1: 51,426

2.2: 52,205

2.5: 54,037

3.0: 57,888

here were my assumptions just to make it simple: More Assumptions: All children born are conceived and born in wedlock. Zero infant mortality (everyone lives to adulthood). All heterosexual marriages are monogamous and faithful. Live expectancy mirrors that of contemporary USA. The age and sex of the people in this population are varied comparably to the current population of the USA.

So the real #s would be much lower because we have infant deaths, etc.

So what would be the kids/couple to sustain equilibrium? 2.2 does not seem to cut it either. I'll try to find out.