World Population imbalance between the sexes

[Lissa]

The Bin Laden thread turned into a look at the population imbalances in India and other developing areas of the world and it got me to thinking. Proposed population growth for the world is projected to hit 9 Billion by 2050, but I've also heard about the male to female imbalance ratio in China due to the one child law. So I decided to do a little search on wikipedia and found the following article:

Human Sex Ratio

Something to note is the various imbalances throughout the world. As can be seen, most western culture countries show an imbalance towards females while certain other cultures show imbalance towards males while others show no imbalance what-so-ever.

The one hypothesis I can see for why western culture has so many females to males could be attributed to wars, in the US the US Civil War and World War II having significant effects on the male population (600,000+ killed in the US Civil War and several hundred thousand in World War II) while Western Europe (and Australia/NZ) saw a whole male generation wiped out during World War I, the Russian and former Soviet countries seeing a large drop due to Stalin killing a proported 50+ Million of his own people, and the Bushido code professed by Japan (win or die).

Now with China and India, two countires that also took part in World War II, there is a cultural propensity for having a male child to carry on the family name. In the case of China specifically with it's one child law, this has led to female children being placed in orphanages and then being adopted out of country most of the time (pre-mid 80s) or abort (post mid-80s) until recently where the Chinese have cracked down families trying to get rid of female children. Then there is the loss of life during the Cultural Revolution (on the order of 30 to 40 Million). Now the article professes the ration of being 1.19 Males to Females, but this number comes from the Chinese government who is known to hide facts to make things look better. I have heard personal accounts of the ratio being much, much worse with number more like 1.5 or even higher (I've heard as high as 2 to 1 in worst case senario) Males to Females. Now India does not have the 1 child law like China, but there has been a push for something similar to reign in the rampant population growth there as well, but again, the culture wants males over females.

Another aspect to look at as well is the epidemic of AIDS in Africa where literally in some nations, 20% to 25% of the population is infected in some way.

So given this information, I'm not sure I can truly see the global population hitting the 9 Billion mark, there will start to be a definite slow down in the areas where population is growing quickly (China this is already occuring, but will likely also be seen in India and Africa due to other factors).

Looking at China specifically, with 10% of the population never procreated (using China's numbers) and the other 90% only replacing one of the two parents, this means that China's population will start to fall significantly over the next 3 to 4 generations (the next generation only reaching roughly 500 Million with the follow on generation likely being around 230 to 250 Million for the following generation and finally 115 to 125 Million for the generation after that assuming the 1 child law stays in effect). In 60 years, China's population should be below what it is now. The US is projected to only add 150 Million to the 300 Million already in the US now by 2050 and Europe will likely see similar numbers as the US (roughly half of the present population being added to the numbers now). To me, the 9 Billion number isn't looking quite as realistic given other factors (we may have a cure for AIDS by then, we may have too little farm land to feed populations causing starvation, we could have wars over resources as it's already being seen concerning water flowing through the Tigris and Euphrates rivers through Turkey, Syria, and Iraq, the Yellowstone Caldera might erupt taking out a large number of the population in the US and Canada immediately with a large amount of people dying due to a volcanic winter due to the ash cloud created much like Krakatoa, and other aspects).

Is anyone else somewhat skeptical about the 9 Billion population figure?

[--Pete]

Hi,

The one hypothesis I can see for why western culture has so many females to males could be attributed to wars, ...

Actually, there is a natural propensity for females in human reproduction. I think the odds are like 41 to 40 for a zygote to be female. Makes sense in a genetic sense. To maintain a population, it takes females. A male can engender many children, a female relatively few. And giving birth is the biggest differential danger during the fertile years (which, until comparatively recently was the whole of the life expectancy).

Is anyone else somewhat skeptical about the 9 Billion population figure?

I am. There are many assumptions made to get that (or any other figure) for future populations. Presently, most of the industrial nations would be having a population reduction if it weren't for immigration. China, as you pointed out, is already in a population draw down mode (although I doubt it will be as extreme as your analysis makes it). India and Africa are the jokers in that deck. As long as the culture is based on the concept of high infant and child mortality (thus making having a lot of children a benefit) but the technology is developing towards that of industrial countries (where having more than two or so children causes a population explosion) there will be problems.

A combination of local laws, a strong propaganda campaign, and tying foreign aid to reasonable population control could make the situation go the other way, and a population of 4 to 5 billion by mid century is not impossible.

--Pete

[kandrathe]

The one hypothesis I can see for why western culture has so many females to males could be attributed to wars, ...

Actually, there is a natural propensity for females in human reproduction. I think the odds are like 41 to 40 for a zygote to be female. Makes sense in a genetic sense. To maintain a population, it takes females. A male can engender many children, a female relatively few. And giving birth is the biggest differential danger during the fertile years (which, until comparatively recently was the whole of the life expectancy).

I understand there are also more lethal genetic issues with the Y chromosome resulting in cell death after fertilization. "Thus, genetic drift is an exceptionally strong force acting upon the Y chromosome. Through sheer random assortment, an adult male may never pass on his Y chromosome if he only has female offspring. Thus, although a male may have a well adapted Y chromosome free of excessive mutation, it may never make it in to the next gene pool. The repeat random loss of well-adapted Y chromosomes, coupled with the tendency of the Y chromosome to evolve to have more deleterious mutations rather than less for reasons described above, contributes to the species-wide degeneration of Y chromosomes through Muller's ratchet. // Primate Y chromosomes, including in humans, have degenerated so much that primates will also evolve new sex determination systems relatively soon, in about 14 million years in humans.-- Wikipedia"

And... Yes, men die at a higher rate for many reasons (Male vs Female Mortality Rates). One idea that surprised me from this article was the higher mortality due to disease and parasites related to the immuno-suppressive effect of testosterone. I've seen studies where women are more attracted to the scent (on t-shirts) of men who have a complimentary immune system. In that article, they also discuss how male risk taking behavior affords more mating opportunities, thus perpetuating that trait (fight or flight). Women are most important for ensuring the survival of their offspring, so women tend to be more risk averse (tend and befriend). Beyond physical capability, I think innately we understand that men are more expendable, and this is why we dedicate men to the more dangerous tasks in our society.

Is anyone else somewhat skeptical about the 9 Billion population figure?

I am. There are many assumptions made to get that (or any other figure) for future populations. Presently, most of the industrial nations would be having a population reduction if it weren't for immigration. China, as you pointed out, is already in a population draw down mode (although I doubt it will be as extreme as your analysis makes it). India and Africa are the jokers in that deck. As long as the culture is based on the concept of high infant and child mortality (thus making having a lot of children a benefit) but the technology is developing towards that of industrial countries (where having more than two or so children causes a population explosion) there will be problems.

A combination of local laws, a strong propaganda campaign, and tying foreign aid to reasonable population control could make the situation go the other way, and a population of 4 to 5 billion by mid century is not impossible.

China has taken steps to curb their population explosion, however they now are looking at a time when they will have many more elderly than those capable of supporting them. India is the next large population producer than has no reins on their uncontrolled growth.

...the Yellowstone Caldera might erupt taking out a large number of the population in the US and Canada immediately with a large amount of people dying due to a volcanic winter due to the ash cloud created much like Krakatoa, and other aspects).

We can count on severe natural events that will adversely affect people and their resources, but truly cataclysmic events are fortunately rare. Predicting a supervolcanic eruption in the next 50 years is very speculative. The last VEI8 (Volcanic Explosivity Index) eruption occurred ~26,500 years ago, and the last VEI7 was Tambora in 1815. The last VEI6 was Mount Pinatubo in 1991. Mount St. Helens was a VEI5. The last time Yellowstone went supercritial was 640,000 years ago. It might be due to blow it's top, but it might take a few more thousand years.

[Jester]

A combination of local laws, a strong propaganda campaign, and tying foreign aid to reasonable population control could make the situation go the other way, and a population of 4 to 5 billion by mid century is not impossible.

Not impossible, but only in the case of a complete stop on all reproduction. We have 40 years until the mid-21st century. We have 6.5 billion people on the planet. Mortality is about 0.85% each year, or about 55 million deaths. Assuming zero reproduction, and holding deaths constant*, it would take around 30 years to get population down to 5 billion, and almost 50 to get it to 4 billion. (It would drop like a rock after that, because we'd lose our whole workforce to "retirement" - not that there would be retirement in such a world.) Although maybe that's what you meant by "reasonable population control."

If we assume even a tiny trickle of children, we don't come even close by mid-century. Even 1/5 of our current birth rate would be enough to cut population decline in half, extending our timeline out to 2070 or later to even hit 5 billion.

My bet is, we overshoot to about 10 billion, then crawl back down slowly to 8 or so. Most of India will get over the demographic hump (...) soon enough, and Africa and other parts of South Asia will follow suit in 20-30 years.

-Jester

* There would be an incresing resource surplus from having to raise no new children for the first 18 years, followed by a slow shortage as we ran out of kids to enter the labour force, and lost productive capacity. Also, without new children we would have to subtract infant mortality and deaths in childbirth, which should lower the mortality rate.

[kandrathe]

* There would be an increasing resource surplus from having to raise no new children for the first 18 years, followed by a slow shortage as we ran out of kids to enter the labour force, and lost productive capacity. Also, without new children we would have to subtract infant mortality and deaths in childbirth, which should lower the mortality rate.

I think the shortages due to excessive numbers of elderly is a bit over blown. I believe we will just see shifts in the burden across age demographics. Youth will be in higher demand, but the elderly will fill in where help is needed. These studies do not account for the quality of life changes, and that many of these elderly are fully capable to continuing to be productive well past 70 years old. If given the choice of working or starvation, the result will be more elderly (60 to 80 years old) will continue in the work force. Also, this trend has happened before, during the world wars when we sent off large numbers of healthy young men to be slaughtered on the battlefields.

[--Pete]

Hi,

Not impossible, but only in the case of a complete stop on all reproduction.

Not really. You are overlooking the compounding of change. You are arguing on the basis of a linear model, but the better model is exponential.

For instance, if you assume a constant death rate of 0.85%/year and no births, then in 2110 the projected world population would be approximately 3 billion, the youngest of whom would be 100 years old. Not a realistic result. The problem is that both the death rate and the birth rate depend on not just the population size, but on the population distribution and sex.

A slightly better calculation gives a world population of 4.8 billion in 2050 if each woman has (on the average) only 1 child. An average of 0.6 would give a population of 4.2 billion.

There are actually some counter intuitive things going on here. For instance, the present death rate is artificially low because of the increase in life expectancy in third world countries.

However, I agree with your projections for India and Africa. Your prediction of an overshoot and decline seem most probable. Stabilization at the 8 billion, not so much. Once a "small family" culture has been adopted over most of the world, the next crisis will probably be human extinction. That's the problem with any rate other than zero -- it leads to explosion or extinction.

--Pete

[Taem]

A slightly better calculation gives a world population of 4.8 billion in 2050 if each woman has (on the average) only 1 child. An average of 0.6 would give a population of 4.2 billion.

I'm curious, do any of these projections take into account homosexuality as a factor? There are breeds of frogs that, when their population becomes too large, start having all females. There are other breeds of animals that, also when their population becomes too large, start exhibiting homosexual or even suicidal traits. It's part of natural order I believe, and I doubt humans are exempt. Although, I don't know how much of a difference it would really make in the end.

However, I agree with your projections for India and Africa. Your prediction of an overshoot and decline seem most probable. Stabilization at the 8 billion, not so much. Once a "small family" culture has been adopted over most of the world, the next crisis will probably be human extinction. That's the problem with any rate other than zero -- it leads to explosion or extinction.

Time to start teraforming other planets!

[Jester]

Not really. You are overlooking the compounding of change. You are arguing on the basis of a linear model, but the better model is exponential.

Why? Growth is exponential. Decline is not. More people are born as a consequence of past births. But fewer people die as a consequence of past deaths, not more.

For instance, if you assume a constant death rate of 0.85%/year and no births, then in 2110 the projected world population would be approximately 3 billion, the youngest of whom would be 100 years old. Not a realistic result. The problem is that both the death rate and the birth rate depend on not just the population size, but on the population distribution and sex.

I did say that there would be a massive crash after around 50 years, as everyone reaches the age at which mortality speeds up - 50 and older. In a zero-births model, obviously, the lifespan of the human race cannot be longer than a human lifespan. But even in that incredible case, the massive decline would not even start until around 2040, as the first cohort passes their healthiest age bracket. Up until then, population decline would simply be a stable function of existing cohorts dying off at present rates.

A slightly better calculation gives a world population of 4.8 billion in 2050 if each woman has (on the average) only 1 child. An average of 0.6 would give a population of 4.2 billion.

How? You'll have to show me your calculation - I thought mine was a limit case. No cohort is dying faster, at least until the youngest cohort hits age 20 or so and production slows down, but no new cohorts are being born.

There are actually some counter intuitive things going on here. For instance, the present death rate is artificially low because of the increase in life expectancy in third world countries.

The idea here is what, that there are cohorts of children and young adults in Africa that would have died as infants, but are surviving, and will die in greater numbers later? Maybe so, but I can't see that having a massive impact on world population numbers. Nontrivial, but not game changing.

That's the problem with any rate other than zero -- it leads to explosion or extinction.

That's why we need more space... the final frontier. ;-)

-Jester

---

It's part of natural order I believe, and I doubt humans are exempt. Although, I don't know how much of a difference it would really make in the end.

We only evolve the mechanisms that were selected for. Some populations regularly reach saturation for their environment, and thus, would gain reproductive fitness by controlling that growth.

Has that happened to humans on an evolutionary timeframe? I would say it has not. Infanticide has been our typical response to population pressures, and aside from issues with killing babies, from a species standpoint, it seems to work well enough.

There is no "natural order" from which we can reason in this fashion.

-Jester

[Klaus]

Not really. You are overlooking the compounding of change. You are arguing on the basis of a linear model, but the better model is exponential.

Why? Growth is exponential. Decline is not. More people are born as a consequence of past births. But fewer people die as a consequence of past deaths, not more.

Using numbers for world population distribution that I found here:
http://www.census.gov/cgi-bin/broker

and mortality rates for the US, which I found here:
http://www.data360.org/dsg.aspx?Data_Set_Group_Id=587

(I did no due diligence on the validity of either data set), I wrote a little simulation to calculate total population by year. Additional simplifying assumption: Nobody lives to be 101 years old, and population within each age bracket is evenly distributed (so, if the range was 20-24 years old and the chart said there were 100 people in that bracket, I put them in as 20 for each age). The output:

Code:

2011, pop 6817058007
2012, pop 6780947321
2013, pop 6743420887
2014, pop 6704654628
2015, pop 6664799512
2016, pop 6623965248
2017, pop 6580863349
2018, pop 6536365484
2019, pop 6490646673
2020, pop 6443857022
2021, pop 6396125182
2022, pop 6345931078
2023, pop 6294327848
2024, pop 6241428235
2025, pop 6187334701
2026, pop 6132236077
2027, pop 6074793053
2028, pop 6015883666
2029, pop 5955653466
2030, pop 5894230835
2031, pop 5831729226
2032, pop 5766880188
2033, pop 5700474675
2034, pop 5632656642
2035, pop 5563553686
2036, pop 5493309282
2037, pop 5420672094
2038, pop 5346373353
2039, pop 5270597102
2040, pop 5193504635
2041, pop 5115237519
2042, pop 5034929397
2043, pop 4953123232
2044, pop 4869969448
2045, pop 4785601594
2046, pop 4700274299
2047, pop 4612919077
2048, pop 4524297722
2049, pop 4434567588
2050, pop 4343866337
2051, pop 4252314568
2052, pop 4158859732
2053, pop 4064282383
2054, pop 3968715974
2055, pop 3872279361
2056, pop 3775421253
2057, pop 3677473877
2058, pop 3578963068
2059, pop 3479997498
2060, pop 3380672244
2061, pop 3281070604
2062, pop 3180354190
2063, pop 3079139025
2064, pop 2977500322
2065, pop 2875509885
2066, pop 2773909639
2067, pop 2672030740
2068, pop 2570426329
2069, pop 2469173531
2070, pop 2368338580
2071, pop 2267978271
2072, pop 2167206766
2073, pop 2066393181
2074, pop 1965634311
2075, pop 1865024424
2076, pop 1766270310
2077, pop 1669280788
2078, pop 1574158861
2079, pop 1480899571
2080, pop 1389493103
2081, pop 1299925584
2082, pop 1211126581
2083, pop 1123494972
2084, pop 1037050593
2085, pop 951831884
2086, pop 871106744
2087, pop 794234247
2088, pop 721419516
2089, pop 652450052
2090, pop 587124292
2091, pop 525251086
2092, pop 465472888
2093, pop 407832128
2094, pop 352359962
2095, pop 299100992
2096, pop 253246323
2097, pop 213781326
2098, pop 179801113
2099, pop 150543516
2100, pop 125352281
2101, pop 103662349
2102, pop 84948759
2103, pop 68836343
2104, pop 54963640
2105, pop 43019421
2106, pop 32735702
2107, pop 22620255
2108, pop 13912123
2109, pop 6415742
2110, pop 0

Population drops to 5 billion in 32 years. This is really simple (mortality rates in other countries are obviously different than the US, and overall are going to be higher), but you can see that the decline is pretty linear until the bulk of the population moves into the higher age ranges.

[Taem]

There is no "natural order" from which we can reason in this fashion.

Fair enough, however it seems to me most of these calculations you guys are throwing out assume the majority of the world will in fact procreate. "There are 'x' billion people on the Earth, so in 'y' years, there will be 'z' more." Because of the fact people are living longer, how many of these people do you think are of procreating age? Of these people, how many will simply choose not to have children, or decide to live a homosexual lifestyle and adopt children? I would say in recent years with Gay Marriage on the rise, this trend is only set to grow, not recede, but I don't see any of this factoring into the calculations presented. I understand it's easy to look at common birthing trends, and draw some conclusions, but by taking the population as a whole and deriving some figures seems, well, incorrect. That is what I was hedging at. But perhaps the figures take all of this into account and I just misunderstood the data, in which case, I should shut up now.

[Lissa]

Various comments to various posts, not going to go through and quote everything since I can't look at the site while at work, but here goes.

According to the wiki article I linked Pete, the ratio of male to female births appears to be between 1.03 and 1.07 male to female. I know we tend to see a stable population around 48% male to 52% female, but actual birth ratio according to the article gives the edge to males. Also, for the western cultures and Japan, all countries have seen significant losses in the males due to wars since the mid 19th century into the mid 20th century. This has had to have a definite curtailing of the male population in western countries including Japan.

As to population stability, I've seen figures, using an exponential growth curve that state that given present death rates (this could easily change as humanity becomes more healthy and infant mortality in third world nations and devolping nations falls) that each generation needs to have 2.3 children per procreating pair to maintain the status quo on population levels, ie, no loss of population no gain in population. This means that countries like China and Western developed countries where birth rates are seen to be below 2.3 will see a lowering of the population over time. The more drastic the cut in births per procreating pair, the more drastic the population drop per generation (which is why I showed China dropping below 1 Billion in 3 to 4 generations if the one child law keeps up along with the imbalance seen of having more males to females, about 6 to 5 by China's count, more like 4 to 3 or even 3 to 2 according to other outside sources lowering the number of procreating pairs further).

As to supervolcanism, ie caldera volcanos, Yellowstone Caldera has a known cycle of 600,000 to 800,000 years. Geologists have noted a rise in the park of 6 inches in just 30 years time and 1 foot in the course of one century. The magma is definitely building in Yellowstone, but it is unknown when Yellowstone will go, but we're definitely seeing something going on with a definite increase in activity since the late 70s, early 80s compared the prior 70 years before that. Will Yellowstone go in our lifetimes, who knows, but it definitely is a sleeping giant ready to go much like Krakatoa did some 70,000 years ago which wiped out most of the Genus Homo at the time (scientists know that Species Sapiens dropped to just over 1000 individuals at the time from looking at mitochondrial DNA).

As to decline being linear, I'm not sure I agree with that. There are a lot of factors for the death of population besides general old age. You have to take into account infant mortality in third world and developing nations, you have to account for accidents, there is also the factor of general crime and/or warfare in a population, along with effects of disease, hazards in the environment either through human cause or through natural cause, and finally the amount of resources to produce food. Wealthier nations will show a propensity for a lower death rate while as poorer countries will show a rise in death rate. Again, these affects are less likely to be linear and more likely to be exponential much like birth rate.

[kandrathe]

According to the wiki article I linked Pete, the ratio of male to female births appears to be between 1.03 and 1.07 male to female. I know we tend to see a stable population around 48% male to 52% female, but actual birth ratio according to the article gives the edge to males. Also, for the western cultures and Japan, all countries have seen significant losses in the males due to wars since the mid 19th century into the mid 20th century. This has had to have a definite curtailing of the male population in western countries including Japan.

Yes, but that is before males start dying off...

Infant Mortality (US)
male: 6.72 deaths/1,000 live births
female: 5.37 deaths/1,000 live births

From conception forward, males are more likely to die. You can see the male attrition by looking at age bands;

Age Structure
0-14 years: 20.1% (male 32,107,900/female 30,781,823)
15-64 years: 66.8% (male 104,411,352/female 104,808,064)
65 years and over: 13.1% (male 17,745,363/female 23,377,542) (2011 est.)

Median Age
male: 35.6 years
female: 38.2 years (2011 est.)

Sex Ratio;
at birth: 1.047 male(s)/female
under 15 years: 1.04 male(s)/female
15-64 years: 1 male(s)/female
65 years and over: 0.75 male(s)/female
total population: 0.97 male(s)/female (2011 est.)

Life expectancy
total population: 78.37 years
male: 75.92 years
female: 80.93 years (2011 est.)

I think the risk of child birth is still one of the greater equalizers in male vs female mortality. All in all, I think I'd rather charge a machine gun nest than go through the event of giving birth.

[--Pete]

Hi,

All in all, I think I'd rather charge a machine gun nest than go through the event of giving birth.

I'm with you 100%, especially given how you and I plumbed.

I'm up to my ass in first of the month garbage -- so I'll come back to this in next Monday or so, with a decent model and some explanations. I've got a lot of it bookmarked from a previous discussion (maybe elsewhere).

--Pete

[DeeBye]

All in all, I think I'd rather charge a machine gun nest than go through the event of giving birth.

Some people say that giving birth is the most beautiful thing. It's not. It's really unnerving just how ugly it is. There's a lot of blood, screaming, and people yelling out instructions - sort of how I imagine a Vietnam prison camp might have looked like. There might even be an unannounced appearance by the local feces band, which likes to attend these events.

When the child is borne, it's all covered in different types of goo and is various shades of purple and the head is misshapen. It also sounds like a banshee. But that's not the end of it, because you have now unlocked the secret "placenta level"! There are no words. It was something out of a horror film.

I'm not even going to describe episiotomies and the stitching thereafter.

Watching all of this was bad enough, but the the doctor had the audacity to ask me to cut the cord. GAH! This was giant pulsating vein that connected my son to my wife, and he hands me a pair of scissors and asks me to cut it. No friggin' way. I could not do it.

I wished I lived in the 50's when the father was banished to the waiting room and smoked cigars while pacing nervously.

Oh, and I bet my wife had it pretty bad as well.

[--Pete]

Hi,

Oh, and I bet my wife had it pretty bad as well.

Ah, but she's probably handled it better, for The Female of the Species is much tougher than the male.

--Pete

[kandrathe]

Oh, and I bet my wife had it pretty bad as well.

With my first son, this was the point in the relationship where I decided whatever she wanted, she deserved. My part of the process was very pleasurable. She did all the hard parts.

I didn't find it all that gross, but I had been prepared by many years of practicing veterinary sciences* (assistant to a large animal vet), and taxidermy. I actually enjoy performing surgery and fixing the broken parts of animals. Were it not such an unrewarding field, I would have been a vet.

* one of the grossest things I ever had to do was sew back together the inner working of a sheep whose birth canal became detached giving birth. After sedating the ewe, we had to push everything back in, deliver the lamb (alive). Then make a small abdominal incision reach in and stitch her back together (~100 stitches). My job was to reach in and hold the birth canal in place from the inside. Mother and baby did very well, and a month later you'd never know she had any issues.

[Jester]

Population drops to 5 billion in 32 years. This is really simple (mortality rates in other countries are obviously different than the US, and overall are going to be higher), but you can see that the decline is pretty linear until the bulk of the population moves into the higher age ranges.

Neat - and I believe supports my back-of-the-envelope no-births model. And to correct my earlier statement, population decline *is* exponential, but only on a lag of 60ish years. Growth is exponential lagged only 18ish years. But with a lag of 60 years before the 1 year olds start to die faster than average, the world population simply cannot crash exponentially before 2070 or so with ordinary mortality.

-Jester

[Klaus]

Population drops to 5 billion in 32 years. This is really simple (mortality rates in other countries are obviously different than the US, and overall are going to be higher), but you can see that the decline is pretty linear until the bulk of the population moves into the higher age ranges.

Neat - and I believe supports my back-of-the-envelope no-births model. And to correct my earlier statement, population decline *is* exponential, but only on a lag of 60ish years. Growth is exponential lagged only 18ish years. But with a lag of 60 years before the 1 year olds start to die faster than average, the world population simply cannot crash exponentially before 2070 or so with ordinary mortality.

-Jester

For giggles, I added a birthrate component to my model. I didn't want to get too crazy with details, so I put it in as if women were 50% of the population, and each woman had X kids when she was 25 years old. If I set X to 1, the population peaks at around 7.1 billion and drops back to 5 billion after 56 years. If I set X to 1.5, peak is about 7.8 billion (25-30 years in) and drops back to 5 billion at about 85 years. X=2.03 is a very long population decline. X=2.04 is ever increasing population (10 billion after about 300 years).