IB ESS HLTopic 8 — Human PopulationsPaper 1 & 2Core skill~10 min read
How Human Populations Change
A population is not a fixed number. People are born, people die, people arrive and people leave — every single day. Everything in this page is really just one idea written in different ways: what goes in, minus what goes out. Get comfortable with that and the formulas stop feeling like a list to memorise.
📚 What you need to know
A population gains people through births and immigration, and loses people through deaths and emigration.
Crude birth rate (CBR) and crude death rate (CDR) are both measured per 1 000 people per year.
Natural increase rate (NIR) compares births and deaths only. It ignores migration.
Total fertility rate (TFR) is the average number of children one woman would have in her lifetime.
Doubling time uses the rule of 70: divide 70 by the growth rate as a percentage.
World population grew very slowly for thousands of years, then shot up after about 1800.
The growth rate is now falling, even though the total is still rising. Those are two different things.
Two ways in, two ways out
Think of a country’s population as water in a tank. Two taps pour water in: births and immigration (people moving into the country). Two drains let water out: deaths and emigration (people moving out).
Whether the level rises or falls has nothing to do with how big the tank already is. It depends only on which is bigger: the flow in, or the flow out.
Births and immigration are inputs; deaths and emigration are outputs. Exam answers about population change should always come back to these four flows.
Students often forget migration completely and answer as if births and deaths were the whole story. If a question says “population change”, check whether migration is included. If it says “natural” change, migration is deliberately left out.
Birth rates and death rates
Raw numbers are useless for comparing countries. Nigeria has far more births than Iceland every year, but Nigeria also has far more people. To compare fairly, we scale everything to per 1 000 people per year.
Crude birth rate
CBR = (live births in a year ÷ total population) × 1 000
Crude death rate
CDR = (deaths in a year ÷ total population) × 1 000
A CBR of 24 means 24 babies were born for every 1 000 people living in that country that year. A CDR of 9 means 9 people out of every 1 000 died. The word “crude” just means the rate has not been adjusted for the age of the population — it is a rough, whole-country figure.
Migration has matching rates. The immigration rate is the number of people arriving per 1 000 people per year, and the emigration rate is the number leaving per 1 000 per year. Subtract one from the other and you get net migration.
WORKED EXAMPLE
A country of 3 500 000 people records 84 000 live births and 31 500 deaths in one year. Calculate the crude birth rate and the crude death rate.
Step 1: put the births into the formulaCBR = (84 000 ÷ 3 500 000) × 1 000CBR = 0.024 × 1 000CBR = 24 births per 1 000 per yearStep 2: do exactly the same with the deathsCDR = (31 500 ÷ 3 500 000) × 1 000 = 9CDR = 9 deaths per 1 000 per yearAlways write the unit. “24” on its own scores nothing; “24 per 1 000 per year” is the answer.
Natural increase, and how long doubling takes
If more people are born than die, the population grows on its own, without anyone moving in. That is natural increase. Subtract the death rate from the birth rate, then divide by 10 to turn “per 1 000” into a percentage.
Natural increase rate
NIR (%) = (CBR − CDR) ÷ 10
Why divide by 10? Because a rate per 1 000 becomes a percentage (per 100) when you divide by 10. If births beat deaths by 15 per 1 000, that is 1.5 per 100, which is 1.5%.
Once you have a growth rate as a percentage, you can estimate how long the population would take to double if that rate carried on. This shortcut is called the rule of 70.
Doubling time
DT = 70 ÷ growth rate (%)
WORKED EXAMPLE
The same country has a CBR of 24 and a CDR of 9. Calculate the natural increase rate and the doubling time.
Step 1: find the gap between births and deaths24 − 9 = 15 per 1 000Step 2: turn it into a percentageNIR = 15 ÷ 10NIR = 1.5% per yearStep 3: rule of 70DT = 70 ÷ 1.5 = 46.66…DT = about 47 yearsThis assumes the rate never changes, which it always does. Doubling time is an estimate, not a prediction.
Notice that NIR does not include migration at all. A country can have a positive NIR and still be shrinking, if enough people emigrate. Several eastern European countries are in exactly that position.
Fertility and life expectancy
Total fertility rate (TFR) is the average number of children a woman would have over her whole life, if the current birth patterns stayed the same. It is worked out from age-specific fertility rates (ASFR): the births per 1 000 women in each five-year age group.
Total fertility rate
TFR = (sum of all ASFR values) × 5
You multiply by 5 because each age group covers five years of a woman’s life. The answer comes out per 1 000 women, so divide by 1 000 to get children per woman.
The number to remember is 2.1. That is replacement level fertility — roughly the TFR needed to keep a population the same size long term. It is slightly above 2 because not every child survives to have children of their own. Below 2.1, a population will eventually shrink unless migration makes up the difference.
WORKED EXAMPLE
A country records these age-specific fertility rates (births per 1 000 women): 15–19: 12, 20–24: 68, 25–29: 92, 30–34: 74, 35–39: 30, 40–44: 8, 45–49: 1. Calculate the total fertility rate and comment on it.
Step 1: add up every age group12 + 68 + 92 + 74 + 30 + 8 + 1 = 285Step 2: multiply by 5285 × 5 = 1 425 births per 1 000 womenStep 3: divide by 1 000 for children per womanTFR = 1.43 children per woman1.43 is well below the replacement level of 2.1, so this population will shrink in the long run unless people move in.
Life expectancy
Life expectancy is the average number of years a newborn is expected to live, assuming today’s healthcare, diet and safety stay the same. It is a useful summary of a country’s living conditions, and it is one of the strongest signals of where a country sits in its development.
How the world population got here
For most of human history the population barely moved. Birth rates were high, but death rates were high too, so the two roughly cancelled out. From about 1800 onwards, better food, cleaner water, vaccination and basic medicine pushed death rates down while birth rates stayed high. That gap is what produced the famous steep curve.
Population did not explode because people suddenly had more babies. It exploded because far fewer of those babies died.
Growing more slowly is still growing
This is the point most students get wrong. The world’s growth rate peaked at around 2% in the late 1960s and has been falling ever since. It is now around 0.9%. But 0.9% of 8 billion is still roughly 70 million extra people a year — a slower percentage of a much bigger number.
Current projections have the world reaching about 10 billion around the middle of this century, then flattening off somewhere near 10 to 11 billion by 2100.
💡 Exam tip
Learn the definitions word for word. “Define crude birth rate” is a one-mark question you can bank, but only if you include per 1 000 people per year.
Show your substitution before your answer. If the arithmetic slips, examiners can still award the method mark.
Watch the units. CBR and CDR are per 1 000. NIR is a percentage. TFR is children per woman.
If a question asks about population change, mention migration. If it asks about natural change, do not.
Quote 2.1 as replacement level fertility whenever you discuss shrinking populations. It is a cheap, precise detail that lifts an answer.
Distinguish a falling growth rate from a falling population. They are not the same.
⚠ Common mix-ups
Forgetting to divide by 10 in the NIR formula. Writing “NIR = 15%” instead of 1.5% is a very common lost mark.
Mixing up immigration and emigration. Immigration is into the country; emigration is exit.
Treating TFR as the number of children currently alive per family. It is a lifetime prediction based on today’s rates.
Thinking a high crude death rate always means poor healthcare. A country with many elderly people, like Japan, has a fairly high CDR because of its age structure, not its hospitals.
Assuming doubling time is a forecast. It only tells you what would happen if today’s rate froze in place.
Saying “the population is falling” when the growth rate falls. Read the axis label before you describe a graph.
Up next: Managing Population Growth — what governments actually do when these numbers head in a direction they do not want.
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