Don't know much about demography

MIND-BOGGLING MATH

Regarding the low birthrates in various countries, could you please explain the calculation behind your observation that the population of Spain will decrease by 50% over the next generation.
Ernest WoodClinton, Mississippi

MARK REPLIES: I meant what I said – that Spain’s population halves with every generation. That’s to say, Generation A has 1.1 children per couple. That means, as a demographic cohort, Generation B will be half the size of Generation A: two million grown-ups will have one million children. If Generation B reproduces at the same rate, Generation C will be half the size of Generation B and only a quarter of Generation A: that’s to say, two million grandparents will have half a million grandchildren. By the time you get to Generation D, you’re looking at some pretty irreversible math. More to the point, how many of those half a million are going to stick around if they have to bear the economic load of supporting those two million seniors. A significant chunk of Generation B and C will act in their economic interests and skip elsewhere.