I have a suspicion that one of the reasons is that fertility is calculated in a wrong way.
We can’t observe total fertility—we only observe the number of children which was born in a given year. To get fertility we need some assumptions about future average female fertility timing (eg that if they will have children, it will happen between 18 and 40 years old).
However, many females choose to have children older in life, closer to 40 and they have technical means to do it like in vitro fertilizations etc.
Except that the thing which is called the TFR is calculated as the fivefold sum of births per woman-cohort per year, where the cohorts are ages 15-19, …, 45-49 and every summand is observed across the years. Were, say, South Korean population to stabilize without raising the thing called the TFR, it would mean that women give birth in a way unreflected by the statistics, i.e. after reaching 50 years. But there is unlikely to be a boom of that late births.
My point is that a girl who is currently in 20s in planning to have children after 40 - this will not be yet visible in “fivefold sum of births per woman-cohort per year” because woman who gave birth now in 40s are from different generation in which was normal to give birth in 20.
Im other words, females who now 20s are planning to have children at 40 and females who now 40 already had their children in 20s—or decided not to have at all. And this we observe gap in fertility.
My main point is that the increase in life expectancy will solve fertility problem.
I asked Claude Opus 4.5 a similar question. It gave me the following reply:
Claude’s reply
Now I have a comprehensive picture. Let me provide you with a thorough analysis.
The Tempo Effect: Real but Insufficient
Turchin is identifying a real phenomenon known in demography as the tempo effect (or timing effect). The period Total Fertility Rate (TFR) can indeed be artificially depressed when women delay childbearing, because births that would have occurred in a given year get pushed to later years. This is well-documented in demographic literature.
What the Research Shows
The tempo effect is real and measurable:
When women give birth at older ages, this “childbearing postponement results in a decline in the number of births in a given period and therefore depresses the period TFR, even though the number of children that women have over their life course does not change.” Austrian Academy of Sciences
The low Eastern German value of 0.80 TFR in 1994 “was influenced by a change to higher maternal age at birth, with the consequence that neither older cohorts, who often already had children, nor younger cohorts, who were postponing childbirth, had many children during that time. The total cohort fertility rate of each age cohort of women in East Germany did not drop as significantly.” Wikipedia
The South Korea Case Study
South Korea is particularly well-studied. A 2018 analysis found:
“The tempo effect linked to the shift toward delayed childbearing has had a strong and persistent negative influence on period TFRs in South Korea since the early 1980s. Without the shift to later childbearing, period fertility rates in South Korea would consistently stay higher and decline more gradually, reaching a threshold of very low fertility, 1.5, only in 2014.” Demographic Research
But here’s the critical problem: Even with tempo adjustment, South Korea’s fertility would only be around 1.5—still far below replacement level of 2.1. And more recent research shows:
“East Asia reopens the debate on the prospect of lowest-low fertility because it can no longer be explained by tempo effects and shows no signs of reversal. Korea may become the first OECD country to resemble the ‘low-fertility trap.’” Duke University Press
The tempo effect is also diminishing because postponement has limits—you can’t keep delaying indefinitely: “More recently, Korean fertility has been characterized by a diminishing tempo effect and falling first and second birth rates.” DOAJ
Why Turchin’s Hypothesis Cannot Stabilize Population
1. Biological Limits of Late Childbearing
Turchin suggests women planning children “after 40” with IVF could close the gap. But the data shows this is implausible:
“The IVF live birth percentage rate per cycle for each age is: Age 40: Live birth rate of 16%. Age 43: Live birth rate of 5%. Age over 44: Live birth rate of 1%.” Advanced Fertility Center of Chicago™
“By age 43, the chance of a live birth from one IVF attempt is down to roughly 5%. By age 45 and above, success with your own eggs becomes exceedingly rare (often below 1% per cycle).” Ivfcenterhawaii
Even with successful pregnancies, “even after transferring the embryo successfully, up to 70-75% of over 40-year old patients who get pregnant have miscarriages.” MyIVFanswers
A 2025 study confirmed: “Success rates in that age group remain below 5% when using a woman’s own eggs… By 2018, donor eggs accounted for more than half of ART births at age 43 to 44, and more than 90% among women 45 to 50.” Southernminn.com
2. Cohort Fertility Is Also Declining
If Turchin were correct, cohort completed fertility (actual children per woman by end of reproductive years) would be stable while period TFR falls. But OECD data shows:
“For all three birth cohorts [1950, 1960, 1975], completed fertility is in most countries well below the 2.1 children per woman needed for population replacement. For the 1950 cohort, only Czechia, France, Iceland, Ireland, the Slovak Republic and Spain have CCF levels above 2.1. For the 1975 cohort, this only concerns Iceland and the United States.” OECD
“The tempo-adjusted fertility declined over the years in most countries, which indicates that fertility is declining faster than what simple birth postponement would explain.” OECD
3. The Gap Isn’t Large Enough
For the United States, “For older birth cohorts of US women—those born between 1959 and 1987 and who were thus age 33 or older in 2020—observed or projected CFRs at age 45 vary between 2.00 and 2.24 births per woman.” Springer
This means even completed cohort fertility is at or slightly below replacement—and this is for cohorts who have already finished or nearly finished childbearing. It doesn’t account for younger cohorts who may have fewer children.
The Bottom Line
The trends Turchin proposes exist, but they are nowhere near sufficient to stabilize population:
The tempo effect typically accounts for 0.2-0.5 children per woman—not enough to bridge the gap from TFRs of 0.7-1.3 to the 2.1 needed for replacement.
Biological constraints make “catching up after 40” effectively impossible at population scale. IVF success rates collapse after 43, and the vast majority of women will not use donor eggs.
Cohort fertility is also declining, meaning it’s not just a timing illusion—women are actually having fewer children over their lifetimes.
The tempo effect is diminishing in many low-fertility countries because postponement is plateauing (you can’t delay past menopause).
Turchin’s observation is directionally correct but quantitatively inadequate. The tempo effect might mean South Korea’s “true” fertility is 0.9 instead of 0.68—but that’s still a 96% population decline in four generations rather than a 97% decline.
Claude’s result of deep research is here. But neither answer implies that your proposal is close to being enough to stabilize the population.
EDIT: Or I could’ve quoted Zvi’sfourpreviousoverviews, of which the second contains things like this and the third contains things like this.
I think it is correct in static, but doesn’t account the future changes of fertility technology and also that opinions of younger people may change. I never wanted to have children before 30 up to the level where I was thinking about things similar to vasectomy.
Imagine a world where people live and are fertile until 100 years old and also their value system randomly fluctuate.
I have a suspicion that one of the reasons is that fertility is calculated in a wrong way.
We can’t observe total fertility—we only observe the number of children which was born in a given year. To get fertility we need some assumptions about future average female fertility timing (eg that if they will have children, it will happen between 18 and 40 years old).
However, many females choose to have children older in life, closer to 40 and they have technical means to do it like in vitro fertilizations etc.
Except that the thing which is called the TFR is calculated as the fivefold sum of births per woman-cohort per year, where the cohorts are ages 15-19, …, 45-49 and every summand is observed across the years. Were, say, South Korean population to stabilize without raising the thing called the TFR, it would mean that women give birth in a way unreflected by the statistics, i.e. after reaching 50 years. But there is unlikely to be a boom of that late births.
My point is that a girl who is currently in 20s in planning to have children after 40 - this will not be yet visible in “fivefold sum of births per woman-cohort per year” because woman who gave birth now in 40s are from different generation in which was normal to give birth in 20.
Im other words, females who now 20s are planning to have children at 40 and females who now 40 already had their children in 20s—or decided not to have at all. And this we observe gap in fertility.
My main point is that the increase in life expectancy will solve fertility problem.
I asked Claude Opus 4.5 a similar question. It gave me the following reply:
Claude’s reply
Now I have a comprehensive picture. Let me provide you with a thorough analysis.
The Tempo Effect: Real but Insufficient
Turchin is identifying a real phenomenon known in demography as the tempo effect (or timing effect). The period Total Fertility Rate (TFR) can indeed be artificially depressed when women delay childbearing, because births that would have occurred in a given year get pushed to later years. This is well-documented in demographic literature.
What the Research Shows
The tempo effect is real and measurable:
When women give birth at older ages, this “childbearing postponement results in a decline in the number of births in a given period and therefore depresses the period TFR, even though the number of children that women have over their life course does not change.” Austrian Academy of Sciences
The low Eastern German value of 0.80 TFR in 1994 “was influenced by a change to higher maternal age at birth, with the consequence that neither older cohorts, who often already had children, nor younger cohorts, who were postponing childbirth, had many children during that time. The total cohort fertility rate of each age cohort of women in East Germany did not drop as significantly.” Wikipedia
The South Korea Case Study
South Korea is particularly well-studied. A 2018 analysis found:
“The tempo effect linked to the shift toward delayed childbearing has had a strong and persistent negative influence on period TFRs in South Korea since the early 1980s. Without the shift to later childbearing, period fertility rates in South Korea would consistently stay higher and decline more gradually, reaching a threshold of very low fertility, 1.5, only in 2014.” Demographic Research
But here’s the critical problem: Even with tempo adjustment, South Korea’s fertility would only be around 1.5—still far below replacement level of 2.1. And more recent research shows:
“East Asia reopens the debate on the prospect of lowest-low fertility because it can no longer be explained by tempo effects and shows no signs of reversal. Korea may become the first OECD country to resemble the ‘low-fertility trap.’” Duke University Press
The tempo effect is also diminishing because postponement has limits—you can’t keep delaying indefinitely: “More recently, Korean fertility has been characterized by a diminishing tempo effect and falling first and second birth rates.” DOAJ
Why Turchin’s Hypothesis Cannot Stabilize Population
1. Biological Limits of Late Childbearing
Turchin suggests women planning children “after 40” with IVF could close the gap. But the data shows this is implausible:
“The IVF live birth percentage rate per cycle for each age is: Age 40: Live birth rate of 16%. Age 43: Live birth rate of 5%. Age over 44: Live birth rate of 1%.” Advanced Fertility Center of Chicago™
“By age 43, the chance of a live birth from one IVF attempt is down to roughly 5%. By age 45 and above, success with your own eggs becomes exceedingly rare (often below 1% per cycle).” Ivfcenterhawaii
Even with successful pregnancies, “even after transferring the embryo successfully, up to 70-75% of over 40-year old patients who get pregnant have miscarriages.” MyIVFanswers
A 2025 study confirmed: “Success rates in that age group remain below 5% when using a woman’s own eggs… By 2018, donor eggs accounted for more than half of ART births at age 43 to 44, and more than 90% among women 45 to 50.” Southernminn.com
2. Cohort Fertility Is Also Declining
If Turchin were correct, cohort completed fertility (actual children per woman by end of reproductive years) would be stable while period TFR falls. But OECD data shows:
“For all three birth cohorts [1950, 1960, 1975], completed fertility is in most countries well below the 2.1 children per woman needed for population replacement. For the 1950 cohort, only Czechia, France, Iceland, Ireland, the Slovak Republic and Spain have CCF levels above 2.1. For the 1975 cohort, this only concerns Iceland and the United States.” OECD
“The tempo-adjusted fertility declined over the years in most countries, which indicates that fertility is declining faster than what simple birth postponement would explain.” OECD
3. The Gap Isn’t Large Enough
For the United States, “For older birth cohorts of US women—those born between 1959 and 1987 and who were thus age 33 or older in 2020—observed or projected CFRs at age 45 vary between 2.00 and 2.24 births per woman.” Springer
This means even completed cohort fertility is at or slightly below replacement—and this is for cohorts who have already finished or nearly finished childbearing. It doesn’t account for younger cohorts who may have fewer children.
The Bottom Line
The trends Turchin proposes exist, but they are nowhere near sufficient to stabilize population:
The tempo effect typically accounts for 0.2-0.5 children per woman—not enough to bridge the gap from TFRs of 0.7-1.3 to the 2.1 needed for replacement.
Biological constraints make “catching up after 40” effectively impossible at population scale. IVF success rates collapse after 43, and the vast majority of women will not use donor eggs.
Cohort fertility is also declining, meaning it’s not just a timing illusion—women are actually having fewer children over their lifetimes.
The tempo effect is diminishing in many low-fertility countries because postponement is plateauing (you can’t delay past menopause).
Turchin’s observation is directionally correct but quantitatively inadequate. The tempo effect might mean South Korea’s “true” fertility is 0.9 instead of 0.68—but that’s still a 96% population decline in four generations rather than a 97% decline.
Claude’s result of deep research is here. But neither answer implies that your proposal is close to being enough to stabilize the population.
EDIT: Or I could’ve quoted Zvi’s four previous overviews, of which the second contains things like this and the third contains things like this.
I think it is correct in static, but doesn’t account the future changes of fertility technology and also that opinions of younger people may change. I never wanted to have children before 30 up to the level where I was thinking about things similar to vasectomy.
Imagine a world where people live and are fertile until 100 years old and also their value system randomly fluctuate.