Reading modern fiction, especially science-fiction, but also a lot of fantasy, will lead one to believe that modern people are incapable of doing math. They seem incapable of seeing that while one plus one may equal two, two cannot remain two in most circumstances.
Let me be a little more clear. I frequently read science-fiction books that involve colonies. Sometimes rich colonies and sometimes poor colonies. And yet, the author of the science fiction book seems incapable of understanding that one cannot have a colony in space with a reproduction rate of two. In other words, barring polygamy, and a polygamy that involves some kind of genetic defect, where women don’t actually bear boys very often, the average woman in a growing colony cannot have two children. In any society where the average woman has two children, the society will gradually sink into nonexistence. It will certainly not populate a colony.
Population growth with a given fertility rate, assuming 10% loss before reproduction.
And then there is the situation where there is some kind of genetic issue that provides some kind of favourable result. Pretty common sci-fi plot. And so, for example, I just read a book, actually a series, where there was one group of people who was only able to pilot spaceships because they had a genetic advantage over the rest of the population, which made them capable of making the jump. Some of them had it more or less, and one particular group of people had it much more than everyone else, and they called it magic, and it used runes… But the point was there was this genetic benefit which was of vital importance to the entire population, and yet we hear absolutely nothing about these people having lots of children. Like, lots of children.
But in order to populate all of the ships and other jobs, which this genetic benefit made possible, those people with the benefit would have to begin reproducing at the earliest age and produce the highest number of children. They would have to be doing practically nothing but reproducing and doing their ‘magic’ job. Because this, too, was a situation of colony planets. This, too, was a situation where the population was theoretically expanding into new world after new world. And this was a situation where they were quite possibly at war. And yet the author seemed incapable of understanding that this would mean that they had to have lots of children! Everyone (since it was a set of colony planets), but especially the main (magic) characters!
I believe that the actual problem is that modern people are incapable of thinking rationally about the process of having children. I was once critiquing a book which took place in mediaeval Scotland. The first part of the book involved a romance and a marriage. And then, the author skipped two years into the future to deal with the next plot point. When I was critiquing the book, I asked the author why it was that the woman, her husband, and their whole village were not upset about her infertility. Because in those two years, she neither had any children nor was pregnant. The author was shocked at my comment and replied that the woman wasn’t infertile!
But of course, according to any medical definition, the woman was infertile. The book gave no hint of there being any problem in the marriage, so presumably, the couple were still having sex frequently. And a couple will have sex more often during a woman’s fertility, because that’s the way a woman is designed. And if a woman and a man have sex for two years in a society without birth control (and where she wouldn’t want to use it anyway!) and the woman does not become pregnant, then, technically, that woman is infertile. And certainly, in mediaeval Scotland, all of their friends and neighbours, and the woman themselves, would certainly be thinking that even after the first few months of her not getting pregnant.
I am open to the idea that all of these mistakes are caused by ignorance. But I don’t see how this ignorance can be so profound and so pervasive. It is at the point now where I would be exceedingly shocked to read a book which actually got fertility rates anything close to correct. Assuming that our government education system has not failed us utterly completely, the math is not actually that difficult. It seems to me that what is difficult is the cognitive dissonance. It seems to me that our modern people have been taught that having too many children is an active evil, and so they are incapable of wrapping their minds around the idea that having more than two children might actually be a necessary mathematical issue in a book that they’re writing or even a story they are reading (I was the only person to point out the problem in that book).
It might seem at first blush that this is a trivial problem. A minor question of a plot hole in a certain kind of book. But I think it is much more serious than that.
If there is anything in our lives that literally prevents us from being able to do the simplest mathematical analysis of any situation, that is a serious problem. It means that there is something that’s going to prevent us from finding out the truth in one or more given areas. This particular area may merely be the plot of a book, but in other parts of our lives, would these serious philosophical issues prevent us from even being able to do basic math?
Our society has rejected children to the extent that they believe in fairy tales. Tales of colonies happily populating themselves with a fertility rate below replacement. Tales of a healthy couple happily having sex for two years without the wife getting pregnant. Tales where the population pyramid can become a diamond, a square, or a circle at a whim, and the society can just continue happily along.
You are absolutely correct. But it's even worse. The fertility rate required for populating a new colony would have to be higher than 2.2, probably much higher. Because of course, in most sci-fi or fantasy novels, there are wars or enemies or illness or other factors that lead to people dying that would imply the necessity of higher fertility to replace the population. Plus, of course, a colony on a new world would imply that they would run into things they couldn't have predicted that would affect the survival rate of infants and children, totally apart from potential effects on fertility itself. It is extremely rare (I think) to find sci fi or fantasy that takes fertility and child-rearing seriously.
The "two years without getting pregnant" is a bit of a new angle that I don't remember from about a decade or two ago before I stopped reading sci-fi, before I picked it up again. Back then, they always had magic birth control, for fantasies, or technologically perfect birth control, for sci fi. I didn't realize they'd moved on to believing that healthy couples didn't get pregnant at all.
You are right that people would start worrying if a woman didn't get pregnant within a couple of years. People would start looking at whether she was getting enough food, or high enough quality food, or whether she needed more rest, or whether she was over-worked, or under too much stress. All of those factors affect ability to achieve or maintain a pregnancy.
All of my writing takes those factors into account. That's why the "married" soldiers in "Cloak and Stola" talk about whether their wives have "caught" yet (gotten pregnant) and deal with their wives having obvious stress and probably nutrition caused delay in achieving pregnancy. And women regularly miscarry as well.
You might want to check out Laura Montgomery's colonization series "Martha's Sons." People have large families, and the requirements of raising families are taken seriously. As are the actual issues with terraforming planets.
https://www.amazon.com/Marthas-Sons/dp/B081QLX6BK?ref_=ast_author_dp
The prevalence of sci-fi and fantasy stories which are just The Current Year But With Spaceships/Dragons is dismaying. It indicates both a collapse of creativity and a decrease in intellect. Population dynamics is something that even popular franchises do not understand at all. Take, for instance, the size of militaries quoted for supposedly highly militaristic evil empires embroiled in galaxy-spanning war. One Star Wars book said the Empire had 10,000 Star Destroyers (read: capital ships) at its height, which is much less than one per inhabited star system. An entry in an old Warhammer 40,000 Codex describes a particular planet as having a planetary defense force of "8 million men...as well defended as any in the Imperium." Really?! Who knew that the Galactic Empire and the Imperium of Man were decidedly under-militarized compared to present-day Sweden.