I understand that many women will not have kids or will have only one kid, or that child mortality is involved but still a fertility rate of 2 means that ON AVERAGE every woman will have 2 kids. This means that every woman and man will be replaced, including the children that die young if the rate of 2 lasts (the newborn females will also have on average two kids). So why isn’t a fertility rate of 2 enough to replace the population?
In: 701
If you view replacement rate through a different lens, it’s basically trying to answer: *How many children should a person capable of giving birth give birth to in order to produce at least one offspring that will be capable of giving birth themself?* To put it more bluntly: *How many live births should a fertile woman expect to have to produce a girl who’ll grow up to be able to give birth to new children?* Not only does this question depend on the probability of giving birth to a girl, but it also depends on giving birth to a girl who will also grow to reproductive age (and thus become the next generation’s “person capable of giving birth” in the question). So it would make sense that this probability will be less than 0.5 and that the answer will be slightly more than 2.0. And yes, giving birth to a girl that will die too early to have children herself is still considered giving birth.
A non-zero number of children born will die before reproducing themselves, be infertile, or willingly refrain from having children for any number of reasons.
Another nuance that I haven’t seen anybody mention yet.
So I ran a simulation for a population growing for something else and got perterbed when my population kept collapsing with a replacement rate of 2 and nobody dying before reaching reproductive age.
What I realized is it you average 2 children exactly then not exactly half will be male and female. Ignore that more males are born and we assume 50% chance just through random deviation you’ll either have more males or females way time.
If they pair up and average 2 children then there will be some single people of the majority sex who cannot find a mate.
503 makes, 497 females means only 497 reproducing couples in my simulation. So I needed a number just shortly greater than 2, wasn’t exactly 2.1 but close and then it stayed roughly stable.
I feel you answered your own question in the question but asked it anyway?
2.1 to cover child deaths which I guess wouldn’t be classed as a woman in the average?
Perfect world on 2.0: population of 1000 people, 500 couples, they each have 2 kids on average, next gen is 1000 people. Lather, rinse, repeat.
Real world with 2.0: population of 1000 people, 476 reproductive couples (some homosexual couples, some sterile, some single people, some die before reproducing), 2 kids each, that’s 952 for next generation. Same ratio, there’s now there’s only 453 reproductive couples, 2 kids each that’s 906 next generation. And so on and so on.
Real world with 2.1: 1000, 476 couples, 2.1 kids means 1000 people for next gen.
Some humans die before puberty. Some humans aren’t able to have children. Some humans never will have children through some combination of factors.
That adds up to the missing 0.1 which is a rule of thumb, and not a precise number. Remember, we’re talking about statistics on a national population level.
Population decline is a serious problem that NOBODY is willing to face. The entire rich world is facing birth rates below replacement. Most of the developing world is stagnant at best. The poor countries are the only ones making more humans, and they can’t afford to feed them.
We’re looking at the extinction of the species if we don’t make major changes.
It’s 2.1 is western countries. In lower income countries it’s slightly higher because of higher child mortality.
Not every child born will survive to adulthood and reproduce. People die before they reach adulthood so you need to have a few extra births to offset that.
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