BellaOnline Forums Religion and Spirituality Atheist Definition of Replacement Fertility

Looking at sites which define replacement fertility -

"The level of fertility at which a cohort of women on the average are having only enough children to replace themselves and their partner in the population. By definition, "replacement" is considered only to have occurred when the offspring reach 15 yrs. In The United States and other industrialized countries, a TFR of 2.1 is considered to be replacement level (more than 2.0 to allow for childhood mortality)."

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I wrote to Dr. Muir who is a professor at Oregon State and who teaches classes on this topic. I asked her explicitly if replacement level fertility has anything to do with female infertility or anything else besides infant mortality.

Here is her response -

"You give me more credit than I am due, but I AM expert enough to be 99.9 % certain that replacement level fertility rates do not take female infertility into account. You could check the web site for Population Reference Bureau to see what they say about it?? Check out

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I do not believe the calculations take into account infertility.

Replacement level fertility is the average number of children a woman needs to have to produce one daughter who survives to childbearing age. Replacement level fertility is also described as the total fertility rate required for the population to replace itself in the long term, without migration.

The internationally accepted replacement level is 2.1 births per woman. Replacement level fertility allows for child mortality (children who die before reaching reproductive age) and the birth of more boys than girls. On average, throughout the world 105 boys are born for every 100 girls. The actual replacement level will vary slightly from country to country, depending on child mortality rates. In countries with high child mortality, the total fertility rate will need to be higher than 2.1 births per woman to achieve replacement level.

The total fertility rate is the average number of live births that a woman would have during her life if she experienced the age-specific fertility rates of a given period (usually a year). It excludes the effect of mortality.

You might contact Joseph Potter in the Department of Sociology at the University of Texas at Austin. He specializes in this type of demography. He's on leave ths semester, but you can probably reach him via e-mail at joe@prc.utexas.edu

>Replacement level fertility is the average number of children a woman needs to have to produce one daughter who survives to childbearing age.

So -- a woman who has 5 sons hasn't produced a daughter, and thus has failed to replace herself?

That's the way the measure works, yes.

Yes definitely Lynn that makes sense to me. Total fertility rate is solely about "how many kids a woman tends to have" - regardless of if that is good or bad for the population or how it affects total population growth. It's just about the average fertility for women in an area. It doesn't matter what happens to those babies after they are born (if they die or not). It's just about how many babies she pops out

I will write Mr. Potter right now, thank you for the contact!

And yes it also makes sense to me that in a country where no babies or children die, ever, that you would need to have 2.0 babies born per woman (or 1.0 girl children born per woman) to then end up with a matching adult population. But in a country where 900 babies died before reaching adulthood out of every 1,000 babies live born, that you would need to create MANY more babies "than necessary" in order to guarantee that the appropriate number of them reached sexual maturity and started the next generation.

Yes. When last I checked, the U.S. was the only developed country (out of the 40 countries considered "developed") with a rate above 2.0 (I think we're at 2.1). Most developed countries have sub-replacement rates and an aging population (which is part of the argument overseas governments are using to justify the push for families to have more children - sub-prime replacement rates mean fewer bodies, eventually, paying into national welfare/support programs).

Well yes this all then gets into the politics of the replacement fertility rate. If you're not maintaining your population then you can have all sorts of problems paying for those who are not working (i.e. the elderly). However right now the world as a whole definitely has more people than it needs. And we in the US is being swamped with immigrants. So I think at least for the foreseeable future that immigration will balance out places that have a nice quality of life but not enough "workers" because of low birth rates.

There are some places that are really overcrowded - like Japan - where I am glad they have a lower birth rate, I think the short term angst of how to arrange the economy is well worth it given the long term benefit of having fewer people crammed into the cities. Have you ever seen how *tiny* the average apartment in Japan is? We're talking really, really small ...

The definition of replacement fertility is flawed. It does not take into account the deaths that may be occurring because of too many births. For example, place a male and a female mouse into a cage. Put in a set amount of food each day. After some time the cage will have a relatively stable population of mice determined by the amount of food you put in each day. What is the replacement rate? Quoted from above: "Replacement level fertility allows for child mortality (children who die before reaching reproductive age) ... The actual replacement level will vary slightly from country to country, depending on child mortality rates. In countries with high child mortality, the total fertility rate will need to be higher than 2.1 births per woman to achieve replacement level." If we use this definition, then we basically determine the child mortality and from that we can determine the replacement rate. If half the pre-adolescent mice are dying, the replacement rate is 4. Notice how this is absurd. What we really should be saying is that the birth rate determines the childhood death rate. If the mice crank out 10 children on average, then in this cage, 8 of 10 children will die. If they produce 6, then 4 of 6 will die. The proper definition for Replacement Rate, what I call the Real Replacement Rate is the lowest possible birth rate that achieves the same population. To determine this for the mice example, first take out half the mice to get them significantly below what can be maximally sustained, then limit them to an average of 2 children. If a child dies, then allow them to have another to replace it. This method properly eliminates the deaths due to over breeding which should not be allowed into any replacement rate. This is not an insignificant difference. Demographers and scientists are blissfully unaware of deaths due to over breeding. We assume the planet and whatever country we are examining can provide for many more humans thus the deaths due to over breeding are nonexistent. This is a horrible assumption.