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Comment by esrauch | Hacker News Reader

When you say "birth rate is falling" its effectively the second derivative on "number of people" right? As in, if we have go from 2 children born per person to 1.5 children born per person, we are still growing the number of people.

Even if "children per person" falls slow enough you could even indefinitely increase the rate that new people are added right?

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That's right, but note that the replacement rate depends on mortality rates and any gender-bias for either birth or mortality. Generally birth rates below 2.1 are consider "sub-replacement", but it can be much higher in some locales (due to disease, war, etc). Here are some per-continent population projections:

https://commons.wikimedia.org/wiki/File:World_population_(UN...

(Interestingly, the above chart was created about 10 years ago; I do wonder how accurate it's been.)

I said children per family, not per person.

I don't really understand the distinction: fewer children per family is the same thing as fewer per person right? Like if you have 1.4 children per person on average that means exactly 2.8 children per adult man + woman?

All I meant to point out is that "birth rate falling" doesn't map to "population falling", if the average number of children per couple drops 2.5 to 2.1 then each generation is still larger (and the population still grows exponentially).

Yeah, it has to fall below 1.0 child per person for the population to decline. And it's commonly reported that is happening in developed countries.

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