BellaOnline Forums Religion and Spirituality Atheist Poll - Birth control

Lisa -
I'll try to do this tactfully, because I value your input to the Forum and hope you'll continue to post here; but the fact is you're quite wrong about the mathematics of the replacement fertility rate. Since talking about it in the abstract doesn't suit you, I'll adopt your strategy, but to make the numbers easier, lets consider a population of 20: ten males and ten females. Assuming they all pair up and have kids, then your approach works fine: zero childhood mortality gives a replacement fertility rate of 2.0 per married couple. If one child fails to make it to adulthood, then the 10 couples must have a total of 21 kids for even replacement, which gives a replacement rate of 2.1.
However, not everyone marries. So, if two of those 20 people stay single, the remaining 9 couples must still produce 20 children in order to replace everyone for the next generation.

How would you calculate the fertility rate for this situation, Lisa? Would you divide twenty kids by the ten couples who might have gotten married or by the nine couples who actually did marry and have children? I think you'd be hard pressed to justify the former approach. In the latter case, of course, the "replacement" fertility rate is 20/9 or approximately 2.2, which is exactly the same as if one couple decided not to have children. If childhood mortality occurred, it would be even higher.

Let's take another (not unrealistic) example. Suppose the population is nineteen: 10 men and 9 women. One woman has two children with one man, divorces him and has one child with another man. The remaining eight couples each have two children. The population stays even, right? What's the fertility rate? Are you going to say there are nine couples (r=2.11...) or ten couples (r=1.9) or are you going to define it somehow as 9.5 couples to get a fertility rate of 2.0 ? Any way you slice it, the value that you calculate for "replacment fertility rate" has nothing to do with childhood mortality!

Finally, let's be even more current and assume 9 men and 10 women. Nine of each pair up and one of the nine married men cheats on his wife and impregnates the tenth unmarried woman. Same questions: how many couples produced those nineteen children and what's the fertility rate?


The point is, Lisa, that in order to have a stable population, couples who have children must make up exactly for a number of factors in addition to childhood mortality, including infertility, couples who decline to reproduce, people who never marry, single mothers (who have children by married men), etc. You're right that the replacement rate is an average, but your fixation on mortality as the only relevant variable is simply wrong.

My original point was this: in the normal course of things, some couples will have more than two children (for any number of reasons.) Ideally the total number of children will work out to be just the right amount to replace the population evenly by the time those who survive get around to reproducing. However, if people begin looking at the replacement rate itself (whatever it is) and using it as justification for having more than two kids, the result would almost certainly be excessive population growth.

I hope I've convinced you by now, but, if not, you can either ask your dad about it or assume that you're right anyway. It's not worth getting into an argument.

By the way, since I agree that intelligence depends on both genetic and environmental factors, is there some reason you bring that up again? What am I missing?

Thanks again for keeping things interesting.

I think my main point of contention was that you were saying the 2.1 was a maximum for a woman, when it had to be an average. There was no way the human population could survive at a break-even rate if it was a maximum. I'm glad we have cleared that up.

As far as people who chose to have more than 2.1 kids, since we're agreeing that half of the people out there DO have more than 2.1 kids, it really doesn't matter why they make that choice - for family history reasons, for farm labor reasons, for liking-to-be-pregnant reasons, etc. Heck, in modern times far *fewer* people are having larger families than ever before in the past. So I really don't think we have to worry too much about them looking at statistical averages as being an important factor in their choice. We have made huge, huge progress in this area and it's one of the least of my worries. The Atlantic ran an interesting article making this very point.

But again, if we look at the year 0BC or the year 1000AD or the year 2000AD, the replacement child rate is completely independent of what women ARE having in terms of family size. It doesn't have anything to do with women choosing to have 1 kid or 100 kids. It is an independent statistic of "how many children does the average woman need to create right now to keep a population steady". Again it's an average. So you take the output of a woman and figure in mortality. Remember, we're talking about a birth rate here. We're not talking about a marriage rate or infidelity rate or sex-having rate anything else. So we need X number of babies to be born in order to guarantee the next generation. So we are starting from the "baby that is born" stage.

If we were talking about "number of sexual encounters to guarantee a next generation being stable" then we would get into accounting for fertility and so on. Heck then we'd get into accounting for birth control

As a final note the actual statistics are 2.1 children per *woman* not couples - it takes out any issues of infidelity, multiple partners, marriage, not marriage etc. It is solely about creating babies at a rate that we have enough adults in the next generation.

Here is the Wikipedia entry on this -

"Replacement fertility is the total fertility rate at which women would have only enough children to replace themselves and their partner. By definition, replacement is only considered to have occurred when the offspring reach 15 years of age. If there were no mortality in the female population until the end of the childbearing years (generally taken as 44 or 49, though some exceptions exist) then the replacement rate would be exactly 2, but in practice it is affected by mortality, especially childhood mortality. The replacement fertility rate is roughly 2.1 births per woman for most industrialized countries but ranges from 2.5 to 3.3 in developing countries because of higher mortality rates.[1] Taken globally, the total fertility rate at replacement is 2.33 children per woman. At this rate, global population growth would trend towards zero."

Lisa -

You're right. Referring fertility rate solely to women eliminates most of the social factors that I mentioned, although not things such as infertility and decisions about childbearing. I think our disagreement comes down primarily to a semantic distinction. Most websites mention only childhood mortality as a determinant of replacement fertility rate, since that varies the most from culture to culture. This may have helped to convince you that it's the only factor.

For a different definition that includes additional variables, you could consult About.com, which states:
"In developed countries, the necessary replacement rate is about 2.1. Since replacement can not occur if a child does not grow to maturity and have their own offspring, the need for the extra .1 child (a 5% buffer) per woman is due to the potential for death and those who choose or are unable to have children. In less developed countries, the replacement rate is around 2.3 due to higher childhood and adult death rates." (emphasis added)

Since you and I have pretty much kicked this topic to death, this will be my last post on the matter. I'm much more interested in seeing where you and the others decide to take this thread.

Having worked at About.com myself for many years, I have to say I am far more likely to trust Wikipedia than a random About.com guide to be accurate in this area

Both sites say the figure is 2.1.

Wikipiedia says the 2.1 is about childhood mortality alone

About.com's guide says the 2.1 is about childhood mortality **plus infertility and other factors**

Both can't be right. The numbers would be different if you figured in other factors.

Also again, this is about babies that are born. So if they're born then infertility is already excluded as a factor. Those babies aren't born. Again it's not about conception rates. It's solely about birth rates. You need 2.1 babies to be born, to exist in "baby form" in order for 2.0 to reach adulthood. So it can't have anything to do with infertility. It could have to do with rates of genetically damaged women having women who therefore have birth defects and have less of a chance of surviving to adulthood. But it is solely about a starting roster of X babies in order to have an ending result of Y adults. It doesn't matter who had those babies or why or anything else. It's just a mathematical count of the # of babies you need to start with, however they got there.

Here is the official definition from the Population Reference Bureau which focuses on this topic:

"In general terms, a two-child per woman average is considered replacement level�the fertility level at which each couple is replaced in the population by their own children, yielding zero population growth. It must slightly above 2 to account for mortality.

In more specific terms, maintaining replacement-level fertility requires that each couple produce a daughter who lives long enough to have her own children. The average number of children per couple, then, must be above two to account for the fact that not all children will survive through their childbearing ages. In addition, because 105 boys are born for every 100 girls, an exact 2-child average would not produce quite enough daughters. In the United States, replacement level is about 2.06 (usually rounded to 2.1). In a higher mortality country, replacement-level fertility would be higher � above 3.0 in some countries-- because so more children die before reaching their childbearing ages."

Just a thought -- does that factor in the fact that, although more baby boys are conceived (Y sperm are lighter and thus faster), more girls survive to adulthood?

Well the mortality is definitely about how many girls survive to the age where they can bear more children. So yes that would be directly the rate of how many girls survive, not really worrying about boys because as we know girls can have multiple partners for their children

>So if they're born then infertility is already excluded as a factor.

Just was perusing this thread -- and this jumped out at me today. Technically, it is sterility that is excluded. Infertility is a different animal -- difficulty in getting pregnant, may or may not be possible. Having dealt with both primary and secondary infertility, and managing to have one child, I know that infertility doesn't mean never having a child.

Lisa � You just can�t let this go, can you? Okay �

(1) You say that you trust Wikipedia, rather than About.com? This is interesting considering the fact that Wikipedia can be edited (anonymously) by anyone. I actually contacted the author of the About.com article, who wrote back:

�Throughout my education in population geography courses and the like I had been trained to read the value that way. I understand it to count for all factors that would keep an individual from propagating�. if you look at any demographics or population geography text, it should repeat what I said.�

Did you by any chance check with the author(s) of the Wikipedia entry? Do you even know who they are?

(2) You state that �You need 2.1 babies to be born, to exist in the �baby form� [whatever that means] in order for 2.0 to reach adulthood.� Are you aware that this would require 1 out of every 21 babies in the United States or about 4.8% to die before reaching adulthood! This is absurd. Here are some facts.

(3) In the year 2005 a little more than 200,000 people died at or before age 44.BellaOnline ALERT: For anti-spam reasons, we restrict the number of URLs allowed in a given post. You have exceeded our maximum number of URLs.

Let's work backwards and just take the items one by one.

Item 6. You say the percentage of women who remain childless is 5% - and provided an article link to verify this fact. However, the article you linked to does NOT say that. Rather, it says "Seven percent of U.S. women aged 35-44 are voluntarily childless" - so it's 7% just for voluntarily - not even including non-voluntary.

For childless reasons, we've discussed this specific topic thoroughly already in these forums. The US Census publishes this information. Women who have reached age 44 who have not had ANY children are as follows, by year. It has not been 5% within the past 30 years at least.

.2004 19.3
.2002 17.9
.2000 19.0
.1998 19.0
.1995 17.5
.1994 17.5
.1992 15.7
.1990 16.0
.1988 14.7
.1987 14.2
.1986 13.2
.1985 11.4
.1984 11.1
.1983 10.1
.1982 11.0
.1981 9.5
.1980 10.1
.1979 9.8
.1978 NA
.1977 10.9
.1976 10.2

The raw tables �

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