Yesterday I posted this question, as I had a problem. My people stopped reproducing. They were happy, healthy and well-fed; everything was great about them, except that they behaved as if they read /r/childfree in all their spare time. A few devout families with 3 kids here and there, but the rest... just chilling out by themselves, not reproducing, wasting their 20s-30s, and then dying alone, happy, with smiles on their faces.

But now I know what the problem was! I've built too many houses. I thought it would boost the population, but actually it looks like there's an optimal amount of vacant houses, as a share of existing houses.

Here's what seems to happen: every person has a desire to live alone (which seems to be a random value for every individual, and which also depends on age), and every person has a certain amount of "love" to other individuals of an opposite sex (which apparently depends on randomized "individual chemistry" and difference in their ages, and which also apparently gradually reduces when they live together). These two desires affect when and how citizens leave their parents as they grow up. Some young people fall in love so badly that they leave their parents even if there are no houses available, and become homeless (I've seen that). With vacant houses available, some kids leave their parents at age 1 or even 0, as in this game they can totally take care of themselves at age 0. But most people wait for these "tendencies" or "desires" to overcome a certain threshold (which depends on the real estate situation), at which point they move out.

Now, if you have lots of vacant houses, the "love" desire rarely wins over the desire to live alone, as dwellings are so abundant. In this situation most people don't get married, but just move from their parents to a vacant house. Sometimes they can have an affair, or even marry and divorce real quick, so I had lots of single parents because of that. But a single parent always had one kid, and my population declined. With houses abundant, only about 20% of people actually married; the rest dwelled alone.

If houses are scarce, however, then the desire to move away from the parents and live alone works "in the same direction" as the "love" or "sympathy" they feel to another citizen, so they move together (because probability of moving together is a function of INDEPENDENCE LOVE). Also divorces are more rare (because probability of getting divorced is a function of INDEPENDENCE-LOVE), and citizens end up having more kids. To divorce they have to more out, and with houses scarce it is pretty hard.

Therefore when you population goes down, one of the ways to boost it up may be to, paradoxically, destroy some houses, and decrease the availability of dwellings.

Another option (thanks /u/libererchoisi) is to only use wooden houses, and periodically inspect them. Every time you find a single person, or a single parent of a reproductive age (more than 14 (??) and below 45 (??)), you hit "Upgrade to stone house", which purges them out. Then you immediately cancel the upgrade, but they become homeless, and with some decent probability they move in with their friend, at least for a while. After a divorce they tend to go to their ex, and more often than not they make another kid in the process. This "moral purging" is rather laborious (I ended up having lots of infoboxes for all empty houses open on my screen all the time), but this way you can make them reproduce really fast.

And because both "love" and "desire to be alone" are individualized, it is actually fun to watch it. Like I had this girl in the only stone house I had in the town; a girl with a funny name, like Emberalda, or something like that. She started to date a vendor from a market nearby when she was, like, 17 (he was 30). Every time I purged him from a wooden house, he would move back to her, but then she would kick him out after about half a year of so, staying alone in the stone house again. Then one time she got a hunter. And then again the vendor. Promiscuous she was, Emberalda. Never got pregnant till she was 25, then got a kid (from the vendor), and then she kicked the kid out of her stone house together with her balding boyfriend, so my poor vendor had to raise this bastard alone. (Interestingly, the bastard moved out from him when he was 2, raising an interesting possibility that the character may be inherited!).

Anyways, just wanted to share my experience.