to double anything (money, population, etc.) in 20 years. That's actually pretty close to our average inflation rate over the same time. The equation to approximate how long it'll take to double something based on a given rate of growth is simply: years = 70 / rate of growth.

The more formal equation to calculate the doubling time in years = ln(2)/grwoth rate where ln(2) is the natural log of 2 and the growth rate is a percentage, e.g. 3.5% = 0.035. Likewise, to calculate the average growth rate to double in a specified number of years, use rate = ln(2)/years. If you want to evaluate some other value than simply doubling (say, tripling the original number), substitute that value for the 2 in ln(2).

Using the numbers in your original post, growing from 7 million in 1980 to 19 million in 2001, we get 19/7 = 2.714 growth factor over 21 years.

Rate = ln(2.714)/21 = 0.0475, or 4.75% growth rate over the period. That's very high compared to most of the rest of the world. For example, last year was the first in decades where the number of births in the US exceeded the number of deaths. For a long time, most of our population growth has come from immigration. Likewise, quite a few countries have birth rates too low to sustain their population. When I was stationed in Germany, the government was giving mothers a bonus for every child because the birth rate was too low. It's also very low in Japan and Russia among others.

While on the one hand we're told to "celebrate diversity", on the other hand it's easy to criticize others whose beliefs are different from our own on specific issues like birth control. Fact is, in several cultures large families are still considered desirable and the norm without a lot of coersion involved. In places like Saudi Arabia where it seems large families are effectively subsidized with guarantees of government jobs, well, what else would you expect?