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?
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Simplest answer (also found with a fast google) is that male children are *slightly* more likely than female children so simply producing 2 kids biases you to a population with excess males. 2.1 children tips the odds to get enough female children in to the mix to maintain a population.
If each woman has exactly 2 children throughout the span of their reproductive lifetime, then the next generation will have less people able to reproduce simply because some of the children will die early and remove themselves from the running.
The additional amount above 2.0 is to account for this, which is why in developed countries replacement rates are as low as 2.1, but in developing countries with much higher mortality, replacement rates can be as high as 3.5.
Because you need the two children themselves to reproduce. Unfortunately not every child born reaches reproductive age.
Because sometimes children die before reproductive age or simply unable or unwilling to do so. So if you always only have 2 offspring, some of them will not have time reproduce and you’ll eventually die be up with a dwindling population. It’s a rounding to .1 for simplicity, death rates/loss rates to missed reproductions differ in different populations.
The replacement rate is calculated with respect to women who reach childbearing age. If you calculated it with respect to any female ever born, it would be exactly 2. However, not every female child survives to childbearing age. A replacement rate of 2.1 is consistent with 1 out of every 20 female children not reaching this age.
You might think it’s silly to calculate the replacement fertility rate like this, but the reason to do it is because it separates two pretty distinct issues. The first is the decision/ability of mature women and their partners to have children. The second is child (primarily infant) mortality.
i can think of three factors right away.
1. Because a %age of women dies before they have offsprings. Thus we need slightly more than 2.1 per woman.
2. Because some women arent able to produce children.
3. Because the ratio of men/women is not perfectly 50/50 but more commonly 51% males and 49 females.
Child mortality.
2.0 would work if every 2.0 children then went on to have 2.0 children. Unfortunately, some will die before having their own children. In order to cancel out these shortfalls there has to be a (very slight) over-replacement rate.
That is less than 5% but more than 2.5% and rounding errors then kick in.
Not everyone lives to adulthood and then has both the ability, will and opportunity to reproduce.
Of those 2.1 children some may get run over by truck or die of cancer before they were old enough to get themselves or anyone else pregnant. Some may turn out to be infertile. Others simply gay (gay couples who adopt adopt children born to the rest and thus don’t count for this math.) Some may simply not want to have children for one reason or another. Incels and catholic priests are a thing.
If one in 21 children don’t grow up to become parents themselves you need 21 children to get 20 to replace the 2 parents to even things out.
Because not all people have children.
People die. They have heart attacks, get stabbed, get shot, jump off buildings, burn in fires, drown in lakes, etc. many of which do so before having children. Plus, sometimes people are just born sterile; or just can’t/won’t have 2 kids.
You need an average above 2 in order to make up for those holes. Not much more; the average is still near 2. But you need more than 2.
It needs to be greater than 2 because people will die before having kids. This will cause a population decline that will lead to the extinction of the species.
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