About the model: a teaching simulation, not a forecast. It uses a simplified version of the cohort-component method that statistics offices use: five-year age groups by sex, births to women aged 15–49, and deaths from child and adult mortality. It leaves out migration and uncertainty, uses one fixed age pattern of childbearing, and a stylized mortality curve. The total fertility rate (TFR) is the average number of births a woman would have over a lifetime at current rates; replacement is the TFR at which each generation just replaces itself, about 2.1 where few children die and higher where more do. Policy effects are simplified mechanisms chosen to teach, not measured effect sizes. Stages come from birth and death rates per 1,000: births of 30 or more with deaths of 22 or more is Stage 1; births of 30 or more is Stage 2; births of 17 or more is Stage 3; lower births is Stage 4; deaths above births is Stage 5. Footprint and Earths are the game’s own relative units, not calibrated global hectares: Earths compares the class’s footprint with what its land renews, and going over slows income growth and raises death rates slightly for everyone. Bongaarts, J. (2009). Human population growth and the demographic transition. Philosophical Transactions of the Royal Society B, 364(1532), 2985–2990.  ·  Keyfitz, N. (1971). On the momentum of population growth. Demography, 8(1), 71–80.  ·  United Nations, Population Division. (2024). World population prospects 2024.  ·  Wackernagel, M., & Rees, W. (1996). Our ecological footprint. New Society Publishers.