Compare two groups by the family size they choose (0–4 children). Replacement math (~2.1) stays in the background for sex ratio and childhood mortality.
Inputs
Enter each group's starting population (millions) and how many children families in that group choose to have. Adjust the time horizon if the trajectories haven't yet diverged or crossed.
Time Horizon
1 decade = 10 years. Try 30 (300 yrs) or 50 (500 yrs) if lines haven't converged.
Group A
Group B
Family choice is what people decide: 0, 1, 2, 3, or 4 children.
Why not exactly 2 for a flat population? Even if every family chooses 2, some children are male, some female, and some die before reproductive age. Demographers therefore use a background replacement level of ~2.1 births on average (Population Reference Bureau) — not a choice families pick in this tool.
Model: each 30-year generation multiplies population by (family choice / 2.1). Choosing 2 → slight decline; choosing 3 → growth. Ignores age structure, changing mortality, and migration.