Rakshith Aloori
Understanding Population Dynamics8th February 2025
Let’s try to understand the world’s population dynamics through a systems lens – birth rate, population and death rate.
    The two questions we want to answer are:
  1. What variables does population growth depend on?
  2. Can we prevent population collapse when the fertility rate stays low?
Assumptions
  • The population has an equal distribution of humans of any age.
Notation
  • L: Lifespan in years
  • P: Population
  • F: Fertility Rate
  • C: Childbearing window of women in years
Death RateIf the lifespan is L and the population is P, after the L years, the P population dies. For each year, then, the death rate would be:Birth RateAssuming 50% of the population is women, for every C years (childbearing window), the number of babies added to the population: is the fraction of women in the childbearing windowFor each year, then, the birth rate would be:Question 1: What variables does population growth depend on?For population growth, we need :So, the lifespan has no effect on population conservation or growth. Fertility rate is the only variable.Increasing C (childbearing window) may increase the fertility rate because it gives women more time to have babies.Question 2: Can we prevent population collapse when the fertility rate stays low?
    During population decline, there are 2 paths we can take to avoid a collapse:
  1. Urge people to have more kids, in other words, increase the fertility rate.
  2. See what else can be done if people still won’t have more kids
Path 1 is self-explanatory.
Let’s assume people are not making enough babies and look at path 2 where we assume the . We are trying to make sure (the population that dies each year) is as low as possible.
The relationship is clear. We can prevent rapid population collapse by increasing the lifespan of people.If I plot population P over time for two different lifespans, 80 and 150 years (assuming a population of 10 bn and a fertility rate of 1.2):Population Decline over TimeBut increasing lifespan is a bandaid solution. We have to, at some point, increase the fertility rate and start growing the population. Fix the leaky boat, or else we sink.Our lifespan may also have limits that can’t be solved at the scientific discovery pace we have. The clock is ticking. Increasing the fertility rate is the only long-term solution.
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