Would you seriously argue that people choose to have children as a reasonably optimal selfish way of guaranteeing that they continue to have enough to eat once they're no longer capable of working?
I don't see humans commonly engaging in a lot of decades off long-term thinking,
This isn't exactly long term thinking. If you live in a culture dominated by extended families, you see your grandparents in your home, and later you will see them die and your parents become the oldest generation in the home. You see the same thing in your neighbor's homes. You see the old lady who has no family living in a hovel and depending on her neighbors or the church for basic needs. You see that you and your parents and your children easily care for most needs of your older generation. You don't need to make a long term calculation; you just have to see that normal people have lots of kids -- and that things work out better for normal people.
Parents often devote significant resources to caring for special needs children who are unlikely to grow into good providers.
All the more reason to have a large extended family. These children will grow into adults who continue to need extra support, and there's no reason for parents to support them on their own. The more siblings you have to help out the better.
It's also not totally obvious to me that children are a particularly good investment from a long-term wealth or even a guaranteed income perspective.
This is because you are thinking of wealth as money. For much of the population of the world, and increasingly so as you go back in time, wealth means enough food on the table, enough food in the root cellar to get you through the winter, and enough grain seed to replant + keep you alive a year or two if the crops fail + plus enough to plant again once the famine is over. As long as another set of hands increases productivity, another pair of hands is a good investment.
Parents often devote significant resources to caring for special needs children who are unlikely to grow into good providers.
All the more reason to have a large extended family. These children will grow into adults who continue to need extra support, and there's no reason for parents to support them on their own. The more siblings you have to help out the better.
From a selfish perspective, the correct decision isn't to have more children. It's to kill or disown the ones who not only won't repay your investment, but will actually compete with you for ...
Over and over again, someone says that living for a very long time would be a bad thing, and then some futurist tries to persuade them that their reasoning is faulty, telling them that they think that way now, but they'll change their minds when they're older.
The thing is, I don't see that happening. I live in a small town full of retirees, and those few I've asked about it are waiting for death peacefully. When I ask them about their ambitions, or things they still want to accomplish, they have none.
Suppose that people mean what they say. Why do they want to die?
The reason is obvious if you just watch them for a few years. They have nothing to live for. They have a great deal of free time, but nothing they really want to do with it. They like visiting friends and relatives, but only so often. The women knit. The men do yardwork. They both work in their gardens and watch a lot of TV. This observational sample is much larger than the few people I've asked.
You folks on LessWrong have lots of interests. You want to understand math, write stories, create start-ups, optimize your lives.
But face it. You're weird. And I mean that in a bad way, evolutionarily speaking. How many of you have kids?
Damn few. The LessWrong mindset is maladaptive. It leads to leaving behind fewer offspring. A well-adapted human cares above all about sex, love, family, and friends, and isn't distracted from those things by an ADD-ish fascination with type theory. That's why they probably have more sex, love, and friends than you do.
Most people do not have open-ended interests the way LWers do. If they have a hobby, it's something repetitive like fly-fishing or needlepoint that doesn't provide an endless frontier for discovery. They marry, they have kids, the kids grow up, they have grandkids, and they're done. If you ask them what the best thing in their life was, they'll say it was having kids. If you ask if they'd do it again, they'll laugh and say absolutely not.
We could get into a long argument over the evolution of aging, and whether people would remain eager to have kids if they remained physically young. Maybe some would. Some would not, though. Many young parents are looking forward to the day their kids leave.
A lot of interests in life are passing. You fall in love with a hobby, you learn it, you do it for a few years, then you get tired of it. The things that were fascinating when you were six hold no magic for you now. Pick up a toy soldier and try to play with it. You can't. Skateboarding seems awesome for about five years, and then everyone except Tony Hawk gets tired of it.
Having kids might be like that for some people. Thing is, it's literally the only thing humans have evolved to be interested in. Once you're tired of that, you're done. If some of you want to keep going, that's an accidental by-product of evolution. And there was no evolutionary pressure to exempt it from the common waning of interest with long exposure.
The way to convert deathists isn't to argue with them, but to get them interested in something. Twist them the way you're twisted.