Note: In this post, I lay out a strong philosophical argument for rational and intelligent people to have children. Even if you are not interested in the topic itself, you might find some of the (tentative) mental models presented here useful.
Are you still reading this post after the previous sentence? Then I guess your disposition towards philosophical arguments is probably at least mildly positive anyway. But we'll go though with the argument, with the hope that it might give you some firmer ground to stand on. Let's call this argument "A":
Argument "A". Consider the group of all people who hear and understand argument "A" (the one you are reading just now). Each of these people can strongly suspect about themselves, and about other people in the group, that they are able to follow not just this particular argument, but also other arguments that are similar in structure. Therefore, when this and similar arguments are presented, they establish a baseline of behaviour for all people in the group, and only those people. If that baseline has rules that are beneficial to everyone in the group, everyone understands that the group should adopt them. If on the other hand the rules would be harmful, everyone understands that the group should not adopt them. Everyone can expect everyone else in the group to follow the same logic, and get the same conclusion. It's not possible to game the system: if one person could conclude after hearing argument "A" that the best strategy is to game the system (e.g. play nice until some point, but then defect selfishly when a good opportunity arises), everyone in the group would find and follow that strategy too, and in the end everyone would get worse results than otherwise. Knowing this, each person chooses to adopt rules that are beneficial for everyone in the group, and follow them in good faith. They understand that it is inevitable, and that the rest of the group will have come to the same conclusion.
Have you swallowed that one? Then I'm pretty sure you'll get the other arguments too. There's no need to worry about all those people who failed to read until this point. And that's the beauty of it!
There's a certain pattern that seems to consistently emerge when a human being develops their life (and other) skills. It's not an argument for or against anything, just something relevant that is definitely worth keeping in mind.
Level "1". A person starts with the "intuitive", or "default" skill set in a particular domain, which means they do pretty much what their brain's cognitive machinery is directly equipped to do by evolutionary adaptations. For example, they help their friends because otherwise they'd feel guilty.
Level "2". At some point that person learns to be "clever": they learn to work around or exploit the properties of human cognitive machinery (their own, as well other people's). For example, they come up with hedonism, and don't help their friends unless it's obviously self-serving. This is, again, mostly domain specific, though there's also a broader whole-person pattern.
Level "3". However if the person develops even more, there comes another stage. They might understand that being clever was not serving them well after all, and see deeper reasons behind some features of level "1". Now their intelligence can be used to fill the gaps in intuitive skill, instead of fighting against it. For example, the person now helps their friends because of updateless decision theory.
People's decisions about parenthood follow the same pattern: level "1" position is to follow the natural desire and have children, level "2" is all the people who are clever enough to redirect their energy to other pursuits, and then there's level "3", which is the subject of this post.
An important premise that I'll need for the argument is this: there's significant room for the modern human population to be more intelligent on average, using ordinary means that are already available to evolution. This is a non-trivial claim, and I'd have to do a lot of work to make a good public case for it. Fortunately for me, I can just point to this recent Slate Star Codex post, in which Scott puts together some fairly impressive historical evidence. Quoting a paper on Ashkenazi Jews (the ethnic group of Albert Einstein), he writes:
Ashkenazi Jews have the highest average IQ of any ethnic group for which there are reliable data. They score 0.75 to 1.0 standard deviations above the general European average, corresponding to an IQ 112 – 115. This fact has social significance because IQ (as measured by IQ tests) is the best predictor we have of success in academic subjects and most jobs. Ashkenazi Jews are just as successful as their tested IQ would predict, and they are hugely overrepresented in occupations and fields with the highest cognitive demands. During the 20th century, they made up about 3% of the US population but won 27% of the US Nobel science prizes and 25% of the Turing Awards. They account for more than half of world chess champions.
For details, check Scott's post and the original paper. Assuming that story checks out, we have an example of relatively mild selection pressure towards more intelligence raising the population average by at least 10 IQ points, over a couple of centuries. And that's only the average. The outliers from this new distribution were so ridiculously smarter than the rest of humanity that they (rhetorically speaking) took over all of advanced mathematics and physics, and did it by themselves.
The selection pressure was likely coming from the fact that the Jews held intellectually demanding jobs, and the difference in the number of children of wealthy and poor families:
Jews who were particularly good at these jobs enjoyed increased reproductive success. Weinryb (1972, see also Hundert 1992) comments: “More children survived to adulthood in affluent families than in less affluent ones. A number of genealogies of business leaders, prominent rabbis, community leaders, and the like – generally belonging to the more affluent classes – show that such people often had four, six, sometimes even eight or nine children who reached adulthood. On the other hands, there are some indications that poorer families tended to be small ones
A final word of caution before presenting the argument itself: it's very easy to strawman it, and I encourage you to give it some space before you let it be attacked and torn to shreds by all those other arguments and counterarguments that might be lurking in various corners of your mind.
After the previous section, we suspect that the human population's average intelligence has room for improvement without any major or drastic changes, and that we have in fact seen it improve locally under the right evolutionary incentives. Looking at this, one question seems particularly pertinent: why is it NOT the case that the general population is constantly under such evolutionary pressure, and its average intelligence is already "maxed out"?
The answer, as derieved from the principles of evolution, as well from the example of Jewish families mentioned above, seems fairly simple. In the general population, more intelligent individuals DON'T systematically produce more offspring than the population average. Anecdotal evidence suggests that intelligent people are more prone to look for the meaning of life, and often find it in some domain that is not relevant to reproduction. If they single-mindedly used their genetic advantage to have as many children as possible, they probably could, but they don't want to.
To put it in another way, human higher cognition is only partially value-aligned with evolutionary incentives, and the extent of this alignment decreases with higher intelligence. This has obvious effects on the gradient of natural selection: it's not adaptive for the population average to exceed a certain "safe" level of intelligence, which is low enough so that people don't overthink their motivations. (At the same time, it might be adaptive to have a lot of variation, so that at least some individuals are able to pick up the slack despite the low average.)
I hope it's clear at this point that I'm NOT talking about direct consequences of some group of people outbreeding other groups. A strawman of the argument above might look like this: some people are smart, and some are not, so if smart people have lots of children, the percentage of smart people will tend to increase in the population. But that's NOT what I mean.
In contrast, I'm saying that intelligence is correlated with how much people's ideas and beliefs impact what they actually end up doing. And if the philosophically correct thing to do for smart people is to not have children, then the incentive gradient will forever be such that there can't be very many people who understand and act on abstract reasoning. An argument in the style of argument "A" applies here. We see that the situation is like this, and that we are clearly in the group of people who understand the situation on this level of sophistication. Therefore, knowing that our decision is the baseline of behaviour for the whole group, the only philosophically correct choice is for us to have lots of children.
Since there's also a negative gradient from people who are smart enough to resist their instincts, but not smart enough to follow and act on arguments such as the one presented here, any actual recommendation based on this would probably aim for significantly more children than the population average (e.g. at least 3 children per adult couple).
Disclaimer 1: In case you are actively working to prevent existential risk, it philosophically correct that you make it a top priority, and ignore everything else that would interfere with that work.
Disclaimer 2: All of the above is not mutually exclusive with other reasons to have children. I certainly hope that all parents will have genuine and human feelings for their children, and go into parenthood in a way that makes sense emotionally. This is regardless of how they weighted up their initial reasons, philosophical or not.