Disclaimer: parent of a bunch o' kids.
The question of "should one have children" is very different from "should YOU have children", as "should a randomly chosen LW'er on average have children" is different from "should an American", or "should a human being (including Somalians) have children". Asking the question broadest in scope, even if answered "correctly", yields mostly personally inapplicable results.
Many of the arguments you gave pertain to the generic "should one have children" more so than they do to the readership of your article, thus losing a good amount of relevancy.
Case in point: all of the different heritability coefficients are dependent on the choice of population. As you become a more reflective person with more options open for you to take, heritability coefficients change. It's like asking about the heritability of IQ going off of dog populations, then concluding that different parenting styles have only x or y impact because the dogs' "parenting" barely impacted their litter's IQ. Higher environmental variance leads to smaller heritability coefficients.
Generically determined factors are still useful data points for public policy debates; they are not for personal choices. There is no logical contradiction there. Someone who wholeheartedly embraces polyamory for his/her personal lifestyle may well conclude that society may be better off living majority-monogamic. Atheists may prefer for the masses to retain religiously instilled doctrines to maintain societal stability (as a tangent, do you really want your janitor to question deeply why he should clean up your trash?).
Another point you make is too US-centric. Yes, raising children is an expensive enterprise in most circumstances, but points of contention such as paying for a college education are a non-issue in some societies, such as many European countries ("student" and "debt" don't share the same word cloud).
More fundamentally, there are a number of implicit assumptions skewing the topic: the equation of morality with charity; most any utilitarian would agree that for altruism to be valued it itself must be encoded in the valuer's own utility function, tautologically so. Someone who values personal procreation over "charities" is as moral as the perfect altruist, each fulfilling their respective utility functions.
Much of the "children as economic caretakers when you reach old age" misses the point. In modern societies, the imperative is less on providing material comfort (though that motif is still present, just less so than in comparison to previous ages) and more on "having people around who give a damn about you".
People who (if you do it right) don't need to be bought, or to be entertained using one's public persona, but who have access to your inner thoughts and care about you because you constructed them that way, providing both nature and nurture. Someone to be there, not to pay the bills but to enjoy and celebrate life, and let you share in that experience (and vice versa).
When you do see someone taking care of someone else for extended periods of time, is it typically a friend, or a relative? That may be too generic, but even in our subpopulation I've yet to hear of the High-IQ-Solstice-friends who then move in with each other once one of them loses his edge due to onsetting dementia. Cameraderie and warm fuzzies between friends are nice and all, but concerning their perceived scope are ultimately a fleeting illusion.
To the obvious response of "yea, look how well those parent-child relationships typically work out, check out all the lonely parents in nursing homes", I'd say "correct for the generic case, but these people ain't doing it right":
Just as guns don't grow on trees, neither does parenting. As with optimizing most other human activity, brains help. If you exchanged the Silicon Valley population with randomly chosen humans with innovative products as the yardstick, you'd be quick to conclude that human advancement is doomed and that in any case it's time to climb back up dem trees, once we lost enough weight for the branches not to break. Wrong study sample, especially as a base for your own personal decisions.
When you do see someone taking care of someone else for extended periods of time, is it typically a friend, or a relative?
If you include spouses under "friends", that might be quite common? I would say typically the spouse will contribute more working-hours in total, although children will be helpful in the last few years.
Disclaimer: I am not a parent.
I've seen a bit of discussion here on whether or not to have children. Most of the discussion that I have seen are about the moral case, but there are factors as well. I'd like to talk about three aspects of parenting that I suspect are the main reasons why people choose to have kids or not: the financial case, the moral case, and the practical case (for lack of a better term). The financial case is straightforward - how expensive is raising kids? The moral case has to do with the best use of resources: is it better to divert resources away from having kids towards charity? The practical case has to do with the actual process of being a parent - the effort it takes and the sense of responsibility.
The Practical Case
I suspect that the main reason for why people don't have kids is because they think that kids are a lot of responsibility because:
1) It takes a lot of work and effort to raise children - effort that could be spent on other activities.
2) Great parenting is extremely important for raising well adjusted, intelligent kids that will grow up to be successful and likable adults.
Regarding 1) yes kids do take a lot of time and effort, but that's not necessarily a bad thing - lots of things that are rewarding require a lot of effort, such as learning a language or a new skill. I don't know what its like to a parent so I won't say much more on this topic.
Regarding 2) it is actually far from a settled question whether parenting style significantly affects the kind of person that your child will grow up to be. There has been some discussion here on the effects of parenting on children. The tentative consensus seems to be that within the range of normal parenting, parenting style has only small impact life outcomes pertaining to happiness, personality, educational achievement. That doesn't mean that how you treat your child doesn't matter. Steven Pinker puts it quite nicely:
The message I would take away is not to worry too much about creating an optimal child. Don't worry about finding the optimal set of extra-curricular activities or the perfect balance of authoritarianism and permissiveness. Instead, try to cultivate a healthy relationship with your child and most of all enjoy the parenting process.
The Financial Case
In agarian societies (and most societies quite frankly) children were/are cheap, in some cases free labor and a life insurance policy for when you retire. But in the post-industrial Western world that is no longer the case. For a middle-upper class family, having a child is a very large cost for two reasons: the first is that children cost a lot of money to raise. The second reason is that having a child might hold you back from advancing your career as much as you would have been able to do otherwise. I will focus on the first problem here. According to the United States department of Agriculture, the average cost of raising a child to age 18 was about $241,080 (in 2012 dollars). This doesn't count the cost of college which can exceed $250,000 at elite institutions. I'll assume the $250,000 figure for the purposes of the following calculations.
Assuming that you are able to invest your money at a modest 5% rate of return, this amounts to having to put aside $8887 each year from your child's birth for college only, and approximately $13,000 (2012 dollars) per year on other expenses such as housing and food. That $13,000 per year figure does not account for inflation and in reality that figure would grow each year but this is just to provide a rough ball-park figure. This figure goes up if you have more than one child but the per child cost goes down.
This brings up the issue of whether or not you "owe" your child an all expenses paid college education. I wouldn't rule out only paying partially for your child's college education especially since this calculation assumes only one child. I would be interested to hear more thoughts on this matter.
The Moral Case
Some effective altruists have advanced the idea that having children is immoral because the money spent on having kids would be better spent by donating it to charity. This assumes utilitarianism, and indeed if GiveWell recommended charities were perfect or even pretty good util maximizers then this argument would succeed, since by design whatever they did would be the best use of money under utilitarianism. However, I do not believe that this is the case. GiveWell recommended charities that focus almost exclusively on public health initiatives, and exclusively focus on providing aid to the poorest countries. While a simple diminishing marginal returns argument might suggest that this is the lowest hanging fruit and hence the best use of money there are other things that need to be considered.
As Apprentice points out the heritability of prosocial behaviors such as cooperativeness, empathy and altruism is 0.5, and I think most people here are aware that IQ has a heritability around that number as well and is a pretty good predictor of life outcomes. If you want to increase the number of people in the world that are like yourself, then having children is a great way of doing so. This is particularly important since high IQ college educated individuals in Western countries have fertility rates that are below replacement levels and are some of the lowest in the world.
Rachels anticipates this argument by pointing out than one child is unlikely to produce the same returns as an investment in charity. I believe this is a mistake because it is short sighted. If you stop the utilitarian analysis at one generation into the future then yes having a smart altruistic child will not give the same returns as saving lives through charity, however consequentialism need not be short sighted. If you have more than one child, and/or if your children have children then the returns get magnified significantly - and it is worth noting that intelligent people contribute a lot to society not just through charity but through their work as well. Moreover, the people you would save by donating to charity would also have children and those children would have children all of whom might require yet more aid in the future. Thus the short term gains in QALYs that giving to GiveWell recommended charities provides lead to a long term drain of resources and human capital. And as I have already mentioned, intelligent people already have the lowest fertility in society, I'd rather not see it go even lower.
Jeff Kaufman provides two counterarguments that caught my eye: that this is an argument for sperm donation rather than having children; and that genetic engineering will solve the dysgenic fertility problem. However, sperm banks are already eugenic (in a sense) and it is fairly easy to saturate the supply of high quality sperm. Sperm donation is good idea for highly intelligent individuals (and to my surprise there are actually sperm donor shortages in some parts of the world making it an even better idea), but it is not a substitute for having children - the bottleneck quickly becomes the demand for said sperm. This is certainly a potential area worth investigating as a light form of eugenics, but I don't know of anyone who's trying to market eugenic sperm donation right now. With regard to genetic engineering, I have serious doubts that the field will develop to the point of commercialization in the next hundred years, and I have even stronger doubts that it will be widely accepted and used. While I realize that prediction of the future is very difficult, I would be very surprised if in a hundred years the average Joe will think about having genetically engineered children. Any mention of eugenics already invokes fear in the hearts of most people, and its pretty hard to deny that genetically engineering babies is the scariest kind of eugenics. Human genetic engineering might well solve the dysgenic problem, but I wouldn't bet strongly on that happening any time soon, whereas having children is an almost guaranteed way of helping to solve the problem.