To start the simulation, click the Start button.

To see how the size of the population is changing over time, click the Population tab at the top of the graph. Try to stabilize the size of the population by adjusting the Fertility slider — it's not easy to bring it under control in just a few years!

To see how the fertility parameters work, click the Fertility tab at the top of the graph. The graph showing birthrate by age of mother will change shape in response to the two fertility sliders.

The effects of the three mortality parameters may be seen under the Mortality tab. Public health programs (vaccinations and protection of the water supply, for example) generally improve infant and child mortality. Medical and pharmaceutical technology, on the other hand, generally improve adult mortality.

As countries industrialize and modernize, they undergo a century-long demographic transition from very high birth and death rates to very low. Historically, infant mortality is the first to decrease, follwed by adult mortality and fertility. At the same time, the average age at which mothers have their first child gradually increases.

Japan and many countries of Europe now have fertility rates well below 2.0, which means that their populations will soon begin to decrease. As this happens the ratio of elderly to working-age adults sharply increases, putting severe strains on the ability of each generation to care for its retirees.

Click here for supporting detail on population dynamics, including a glossary of terms, links to quantitative evidence and data, alternative views and opinions, and the workings of the mathematical model.