Pop. Principles

TLDR; The magic number is around 2.3, not the 2.0 you might expect, results in a stable population. This is an exploration of what 'births per woman' means, and how it is measured. Key to discussion about population is population growth, which is most commonly analysed from 'births per woman', which is also called 'fertility rate'. But what is 'births per woman'? Answer: not quite what you might expect, which means a small adjustment is required for interpreting births per woman when predicting population growth.

It seems like the human population has forever been growing, but any analysis makes it clear growth must stop eventually at some level. Perhaps consider that the wealthier each person, the less people the planet can support when answering the question of at what level should growth stop? Do we go for the maximum possible people just before everything collapses, even if average living standards could be far better with a smaller population? When caged hens are farmed for eggs, people advocate for a lower population to allow living free-range for a better existence, yet with people being farmed by multinationals, billionaires and politicians growing the human population to grow the economy, it seems that denser and denser housing and reduced resources per person for human workers is accepted. Most countries seem to agree that global population growth should end but and state that ends its own population growth faces economic disaster.

Why your own life expectancy is greater than the average, and the rise in life expectancy from the just 32 years at the start of 20th to now being over 70 years by the 21st century does not mean what most people think. What has changed is less about ever longer lifespans, and more about people living an increasing percentage of the same full lifespan. Despite the lower life expectancy during earlier times, most famous figures lived to around age 70 not only in 1900 but even 2,000 years ago and Ramesses the Great lived to 90 back over 3,000 years ago! Infants are rarely famous, and as it is the reduction of child mortality that has the biggest impact on life-expectancy, despite the ageing population you don't see as much difference when looking at how long adults live, but we may soon!

From childhood I had assumed human population was growing because the world was not yet fully populated and life was still expanding, but reality is most the recent "garden of Eden scenario" occurred over 500 million years ago, and since then the planet has been fully occupied with a total biosphere of life in fact gradually shrinking. It turns out that no species on Earth can keep increasing population without quicky on geological timescales reaching its population limit. How has the human population kept growing if we have long been at our limit? By raising our limit through accessorised evolution and in turn lowering the environmental limits of other species. We have been, by the same principle of evolution Darwin declared "survival of the fittest" that allows new species to arise and grow their populations, competition from humans has been partially or fully "evicting" other species from their environmental niches. While when there were less humans this had little impact, it has now reached a scale labelled the Anthropocene where, if the human population continues to evolve and grow, other species face decline or face extinction.

Throughout history, although no other species on Earth has experienced such long term overall population growth, even before the recent population explosion, the human population kept slowly growing. Yes, we recently had an unprecedented population explosion, driven by the near elimination of previously tragic rate of infant mortality, but against a background of more gradual long-term growth, many of us may have never even realised we just had a population explosion. But what drove that long-term population growth even before the explosion, and what will now happen as the explosion ends?