A good friend and I were speaking about having children when she said that she and her husband had only two children because they wanted zero population growth. The thinking was
two children to replace her and her husband.
I don’t think that’s right because she will live at the same time her sons are living and will most likely be alive when the two boys have children of their own.
So! In her zero poulation gorwth theory, she had two children, they each have two children to replace them and their wives and now we have two people creating six additional people (sons, two children each) all living at the same time.
She insists I am incorrect. Who is right here - or are we both incorrect?
If suddenly from today everyone only ever had 2 children, then the population would continue to increase (although more slowly than today) for a while for the reason you give above.
Assuming that noone dies before having birth then the population would eventually stabilise, and stay constant. However not every child would live long enough to have children of their own (or might not want to have children), and so the population would actually start to decrease.
You have to take the really, really long-term view, like 50 years. In 50 years Mom and Dad will have both passed on, leaving their two children to be their replacements. If Mom and Dad had had four kids, when they passed on, they would be leaving their two replacements behind, plus two additional persons.
Plus, recall that in a real world situation replacment is actually 2+. There’s always a certain group of people who don’t reproduce at all. So if each couple had two children we wouldn’t level off but start to decline.
I remember reading that, depending on local circumstances, the number of children required per couple to maintain population equilibrium is between 2.1 and 2.4.
Population equilibrium has always been sort of arbitrarily set at 2.1 for an industrialized country.
If every couple had 2 kids, the population would, very slowly, decline (since a small percentage of children would die in childhood and never have kids.)
Pertinent to the OP, you’re forgetting about all the other grandparents and parents of your grandchildren. This is why, I think, it seems to you that two people alone are making these six people.
Actually the whole issue is kind of moot. Recent data suggest that population growth rates are declining worldwide, and that the total world population will finally begin to decline in the second half of this century.