How many births per person would balance out the deaths per person on earth right now? The birth rate on earth is higher than the number of deaths per year, so the population is increasing. At what rate is it decreasing? How would that translate to a birth control policy? i.e. what is the balancing number of births, beyond which the population of the earth increases? I don’t know the appropriate unit to even think about - births responsible for per person per life? I await clarification of my question and a thoughtful set of responses.
The replacement-level total fertility rate (simplifying, but the average number of children per woman) in industrialized countries is 2.1; at this level, the population should stay stable, ignoring immigration. The replacement TFR is higher in countries with lower life expectancies, where, among other factors, high infant and child mortality means that you need to give birth to more children to get the same number of reproducing adults.
The US TFR was actually 2.1 last year, a bit higher than it had been over the previous few years, although the US population has been growing slowly but steadily because of immigration. The TFR in much of Europe is well below replacement levels, as is that of Japan. (A ranking of the TFRs of every country is here.
The global mortality rate is exactly 100%: Everyone who’s born will eventually die. As such, the average replacement fertility rate is exactly 1 per person, or 2 per couple. The commonly-quoted figure of 2.1 only holds if you for some reason throw people who don’t reproduce out of the data set entirely (and if you make assumptions about how many such folks there are), instead of counting them as having zero children.
Suppose I ask a different question, then. If there were to be a world-wide birth control policy, what number of children should any particular couple have in order to match, but not exceed, the mortality rate, world wide?
I thought that it was to account for people that die before reproducing, given that the assumption is that everyone who makes it to adulthood marries and has children.
Another important factor is at what age they have children. If everyone has 2 children at age 18 the population is going to grow a lot faster than if everyone has two children at age 36.
Mortality has held steady at one death per person, but criminality has come down a lot since Cain and Abel.
2 kids per couple works as replacement rate on the assumption that they will all reproduce. Reality might factor in and push this number higher, and if you don’t someone else might pick up the slack, but I feel the OP is asking something along the lines of a world wide reproductive policy.
That is, if every person were to contribute equally to the next generation, how many kids should they have? And I am saying person instead of couple as marriages are not necessary to have kids. When a couple has a kid, they would have to agree on whose quota kid that is, I guess.
I am assuming that this world government would somehow have to consider infant mortality and all that leads to a birth not resulting into another child later on. This would mean that every person should have 1.something kids.
Disregarding the complexity of having fractional children, I think the OP wants to know what is a decent estimate of what that point something should be.
If this is true, why is the population of China (TFR 1.8) still increasing?
Presumably because it still has a relatively high proportion of the population in child-bearing ages. That would be a hangover from earlier periods when:
(1) People had more children, and
(2) A lot of people died from wars and famines.
As the population of China ages, the population increase will stop happening.
You’re asking two very different questions. First, you’re asking what it would take to stabilize world population right now. There are about 56 million deaths per year. To balance that, we need 56 million births.
According to the US Census Bureau, there are 1.54 billion women in the world between the ages of 15 and 45. If 3.6% of them bear a child in the coming year, the world population will remain the same.
Second, you’re asking what it will take to stablize the population long term. That’s a very different question. The fertility rate is extrapolated from births to women who have already reached child-bearing years, and thus the replacement rate is a little bit higher than 2.0–replacement rate is usually considered to be 2.1.
This will ensure that each generation replicates itself. However, this can lead to either a rising or falling population, depending on the proportion of child-bearing women in the population and changes in life expectancy.
No, 2 kids per couple explicitly does not make that assumption. 2 per couple is an average, and the people who have zero are figured into it. If one couple has four children, and another has none at all, then the average is still 2. The only way to get any value other than exactly 2 for the average replacement rate is to use different standards for the parents and children.
No, as others have said it’s a bit more than 2, because some people do not survive to the age where they might start reproducing. They get counted in the total population, but not in the number of “couples”. However, I think single adults, and gay and lesbian couples who don’t do any reproducing, do get counted in the couples.
Correct. From the standpoint of a fertility rate calculation, you are either a woman of child-bearing age, or you are not. Fertility rates are extrapolated from the number of women in each age cohort. There’s no need to make an adjustment for the fact that certain women are infertile, or lesbian, or not interested in bearing children, because these women are alive and make up part of the age cohort and thus get factored into the fertility rate.
However the process of extrapolation requires an assumption that every woman in each measured age cohort will survive to the end of her child-bearing years. Some won’t, so the fertility rate will be a slight overstatement. In addition, some children in the next generation won’t survive even to the beginning of child-bearing years, so a replacement rate greater than 2.0 is necessary.
This is a little counter-intuitive. Sasha dies at age 5, Sarah lives to be 100 but never has any children. Why does Sasha require a greater-than-2 replacement rate, but Sarah doesn’t? Because Sarah lives through child-bearing years and gets counted in the denominator from which the fertility rate is calculated.
Chronos is saying that if you count people who die before adulthood, the average number is exactly 2 for every 2 people. If you don’t want to count people who die before childbearing years, that’s your lookout, not his.
And therefore have zero children, and should be counted the same as anyone else who has zero children. No matter what standard you use, if you use the same standard for parents and children, it’ll work out. In a replacement-level society, for instance, you could say that every newborn girl will have an average of one daughter born, or you can say that every five-year-old girl will have an average of one daughter who reaches the age of five, or you can say that every female of childbearing age will have an average of one daughter who reaches childbearing age, and so on. You only get funny numbers when you put conditions like “…of childbearing age” onto the parents, but not onto the offspring.
Further, if you do that, then what funny numbers you get depend on other factors that nobody ever asks about. If, for instance, medical and safety technology were to advance to the point that every child born were guaranteed to reach childbearing age, then the number would be back down to exactly 2. As it is, in the developed world, we’re already almost to that point: The overwhelming majority of children in the developed world do live to childbearing age. I suspect that the oft-quoted value of 2.1 comes from someone figuring that it’s a little more than 2, and fudging the exact number.
One child per person for replacement (on average, hence premature death doesn’t matter since that simply counts as zero children) is certainly right, but it seems to me that that would only work out to two per couple in a population that’s exactly 50/50 male/female, i.e. all persons can possibly be part of a reproducing couple; if there were only three people in the world, there could only be one couple, and two children would not sustain the population.
Sure. You can say that in a replacement society every woman of childbearing age will have an average of one daughter who reaches the same age, but that’s not the same question as asking how many baby girls (on average) she needs to have to make that happen. And the answer is, more than one. Only a little bit more than one in developed countries, but still slightly more than one.
In demographic terms, it’s referred to as population momentum. China still has one heck of a lot of people in the child-bearing ages. That’s a lot of babies no matter how few each one has.
I interpret this question to mean:
What should the birthrate be, to make the population on earth exactly constant - meaning that world human population would be the same 100 years from now as it is today?
And I am interpreting this answer to mean: if the fertility rate were to be 1 child per person, then the world population would remain constant.
Assuming both of my interpretations are correct (a big assumption), then I think that Chronos’ answer is wrong. The numbers depend on the lifespan of the individual and the age at which a person is giving birth.
I tried to write up a simple example.
Assumptions: we start with one couple A0 and B0. The next generation will be A1 and B1. etc. Each person always has a child at age N and dies at age M.
If the lifespan for each person increases more than the age at which a person gives birth, then the population will increase, not remain constant.
Example:
Yn = age at which people have children
Dn = age of death - Yn
Y1 = 15, D1 = 15 (people have children at 15, die at age 30, 15 years after giving birth)
year 0: Population = 2 (A0 & B0)
year 15: Population = 4 (A0 & B0, A1 & B1) A1 & B1 are born
year 30: Population = 4 (A1 & B1, A2 & B2) A0 & B0 die, A2 & B2 are born
year 45: Population = 4 (A2 & B2, A3 & B3) A1 & B1 die, A3 & B3 are born
(in year 46, medical and social advances: Y2 = 20, D2 = 30)
people have children at 20, die at age 50, 30 years after giving birth
increase in child-bearing age is Y2 - Y1 = 5
increase in lifespan after procreation is D2 - D1 = 15)
year 65: Population = 6 (A2 & B2, A3 & B3, A4 & B4) A4 & B4 are born
year 80: Population = 4 (A3 & B3, A4 & B4) A2 & B2 die
year 85: Population = 6 (A3 & B3, A4 & B4, A5 & B5) A5 & B5 are born
year 95: Population = 4 (A4 & B4, A5 & B5) A3 & B3 die
year 105: Population = 6 (A4 & B4, A5 & B5, A6 & B6) A6 & B6 are born
(in year 106, medical and social advances: Y3 = 25, D3 = 40
people have children at age 25 and live to age 65
increase in child-bearing age is Y3 - Y2 = 5
increase in lifespan after procreation is D3 - D2 = 20)
year 130: Population = 6 (A5 & B5, A6 & B6, A7 & B7) A4 & B4 die, A7 & B7 are born
year 150: Population = 4 (A6 & B6, A7 & B7) A5 & B5 die
year 155: Population = 6 (A6 & B6, A7 & B7, A8 & B8) A8 & B8 are born
year 170: Population = 8 (A7 & B7, A8 & B8, A9 & B9) A6 & B6 die, A9 & B9 are born
year 195: Population = 6 (A8 & B8, A9 & B9, A10 & B10) A7 & B7 die, A10 & B10 are born
year 220: Population = 6 (A9 & B9, A10 & B10, A11 & B11) A8 & B8 die, A11 & B11 are born
etc…
As we see, the earth’s population is increasing.
Currently, about 1/3 of gay & lesbian couples do have children in their household. Some of these are adopted, but many (especially lesbian couples) are their own biological children. So counting them as zero reproduction would not be accurate.
And why would “single adults” be considered as non-reproducing? Current statistics for the US indicate that somewhere over 20% of births are to unmarried (“single”) mothers.