Episode 2: 2.1 Kids: A Stable Population

What does it take to replace ourselves? Are we doing it?


Which scientists say it takes 1 person to replace another?

::polite cough::

Where does the 2.1 come from? Wouldn’t replacement rate fertility be 2.0?

Replacement rate fertility requires each woman to replace herself. According to the CIA World Factbook, there are 107 boys born for every 100 girls. Thus each 100 women need to bear 207 children, on average, in order to produce the 100 girls needed to replace them. Dividing 207 children by 100 women equals 2.07 children per woman, which convention rounds up to 2.1

The population doesn’t decrease that quickly! People stick around.

But not indefinitely. Everyone will eventually die. Medical advances have managed to extend lifespans, masking the effect of low fertility rates on population size for a time. But when the generations that failed to replace themselves begin to die off, the population begins to rapidly shrink. Dramatic reductions in population are now underway in most developed countries. It is a vicious cycle, and one that, because of the scarcity of young people, is very difficult to escape from.

What economic hardships will happen if the fertility rate is too low?

Society is made up of old people and young people. Old people didn’t start off old; they were once young: working, having families, and paying taxes. These young people gradually age until they are old and no longer able to work. When this happens, it is important for them to be fully replaced by a new generation of young people. These young people will in turn work, have families, and pay taxes, which go in part to supporting the elderly population which can no longer support itself.

However, this setup only works if the young are more or less equal in number to the elderly. If the number of workers becomes too few to support the number of elderly, then the whole system faces the danger of a catastrophic collapse. Witness the debate over Social Security.

Why is the replacement rate higher in developing nations?

Many developing nations have very high rates of infant and child mortality. As a result, the total fertility rate needs to be higher in order to offset these losses.

Where are you getting these numbers?

The list of developed countries is taken from the U.N.’s list of countries with very high human development (page 213).

The Total Fertility Rates for these countries are taken from the CIA World Factbook for 2010. Check it online.


Big deal. If future generations need more people, let them have more kids.

Would that it were so simple. When a population decreases in size, the number of potential mothers also decreases. We say that countries with very low birthrates–like Japan’s 1.21 children per woman–are in demographic collapse because each new generation is little more than half the size of the one that preceded it. At this rate, it would take only four generations to reduce the size of population to 10 percent of its initial size. To offset this decline and restore the population to its initial numbers, each woman would need to have 20 children! Hardly a tenable solution.

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