"People in our world live twice as long. I don't know how strongly that's local medicine and sanitation and how much it's just a species difference, but if any of it is the sanitation and medicine..."

"Vaccinations are recent, we don't yet know how much they help with the upper end. They took a bite out of child mortality though."

"In our history the fall in child mortality preceded a very turbulent time, but maybe babies won't start to scale up and things will be better here."

Kib nods.

"The technical problems I'm looking for people to work on are assembly lines, metal mining, and public transit."

"Why public transit?"

"It might be too early for it to be profitable in itself but if it exists then other things can get organized around it and it's a pain to do right if everything's built up around not having it and it contributes a lot to freedom of movement."

"Between cities, in them, or both?"

"Between cities I want railroads and that might have to wait on getting better at mining. Within cities is really important once you have actual cities, which, uh, you don't, yet."

"You caught us, it's made of canvas and paint."

"The designation where I'm from is that under a million is a village and under ten million is a town."

"Seriously?"

"Yep. ...well, if the city boundaries are themselves, like, seven million for historical reasons but the metropolitan area is fifteen million that counts."

"Metropolitan area?"

"Walls stop being a good way to designate the boundaries of cities once people invent good ways of breaking them down. In, say, Lateli, you have a little island in the middle of a river, which is the city proper and only has nine million people living there, but then you have areas on both riverbanks that are fifteen minutes' or less travel from downtown and which it makes sense to administer as part of the Lateli metropolitan area. - we do more governing than your governments tend to, how to designate places for the purpose of administering them is an important problem at home. I don't think that one's a straight advantage, and I'm not in a particular hurry to get it here."

"Do you just - stack everybody on top of each other in huge towers -"

"Yes, we do." He pulls up a picture of Lina on his pocket everything. The buildings are not the focus of the picture but they're there incidentally in the background.

"Wow."

"Cities are good for people. Living close to your friends, close to your family, close to people you might want to date, close to work, with enough kinds of work in the same place that you can change as often as you want without disrupting your whole life. There are places that were bad at cities and they lost to the places that were good at them, because cities are important. But it would be a bad idea to do that here if babies scale with your population - I wasn't kidding about potentially having to kill a hundred million of them a year, that's smaller than the number of births my world has to prevent every year to keep the population stable."

"...how do you estimate how many births you're preventing?"

"Survey people on what they would do if not for population controls. Since we have births the non-magic-appearance-on-hilltop way."

"Fair enough. If you think they tell the truth on surveys, maybe you have a way to do that."

"I'm not a statistician but I think they're pretty good at arriving at real estimates from surveys which individually have various shortcomings. I also don't think people would lie about that especially."

Nod. "I mean, if it's like animals it's probably rough to just... not get married for a long time? Avoid your spouse till you're allowed?"

" - oh, no, we have a way of preventing it when we don't have permission."