IB ESS SL8.1 Human PopulationsPaper 1 & 2~14 min read
How Human Populations Change
A human population is a system, exactly like the ones you met earlier in the course. Two flows add people, two flows take them away, and the size of the population is just the balance between them. Every calculation on this page is a way of measuring one of those four flows.
📘 What you need to know
Inputs that grow a population: births and immigration.
Outputs that shrink it: deaths and emigration.
Crude birth rate (CBR): live births per 1 000 people per year.
Crude death rate (CDR): deaths per 1 000 people per year.
Total fertility rate (TFR): average number of children a woman is expected to have in her lifetime.
Life expectancy: average years a person is expected to live from birth.
Natural increase rate (NIR) = (CBR − CDR) ÷ 10, given as a percentage.
Doubling time (DT) = 70 ÷ growth rate % — the “rule of 70”.
World population was about 1 billion in 1800 and passed 8 billion in 2024.
The growth rate peaked at about 2.1% in the late 1960s and has been falling since.
The UN models the future with high, medium and low fertility scenarios. Medium is the most likely.
A population as a system
Before any of the formulas, get the picture straight. Four flows change the size of a population. Two put people in, two take people out.
Natural increase and total population change are not the same thing. Migration is the difference between them.
The measures you have to be able to calculate
These come up as short calculation questions, so learn the formula and practise the arithmetic until it is automatic.
Crude birth rate and crude death rate
The crude birth rate (CBR) is the number of live births per 1 000 people in a population per year. A CBR of 15 means 15 babies are born for every 1 000 people that year.
Crude birth rate
CBR = (total number of live births ÷ total population) × 1 000
The crude death rate (CDR) works exactly the same way: the number of deaths per 1 000 people per year. A CDR of 8 means 8 people die for every 1 000 in that population each year.
Crude death rate
CDR = (total number of deaths ÷ total population) × 1 000
Migration is measured the same way. The immigration rate is the number of immigrants per 1 000 people per year, and the emigration rate is the number of people leaving per 1 000 people per year.
The word “crude” is doing real work here. It means the rate ignores the age structure of the population. A country full of young adults will have a low crude death rate even with poor healthcare, simply because young people rarely die. Say that in an evaluation question and you will pick up a mark.
Total fertility rate
Total fertility rate (TFR) is the average number of children a woman is expected to have during her lifetime, based on current age-specific fertility rates (ASFR). In developing countries TFR tends to be higher, partly because access to family planning is limited.
Total fertility rate
TFR = (sum of all age-specific fertility rates) × 5
Why five? Because each age band in the table is five years wide. A woman spends five years in each band, so each rate has to be counted five times over.
Life expectancy, doubling time and natural increase
Measure
What it tells you
How to work it out
Life expectancy
The average number of years a person is expected to live from birth, if current conditions such as healthcare stay the same
Read from data; no formula needed
Doubling time (DT)
How many years a population would take to double at its current growth rate
DT = 70 ÷ growth rate %
Natural increase rate (NIR)
The difference between births and deaths, as a percentage. If CBR is higher than CDR, the population grows naturally
NIR = (CBR − CDR) ÷ 10
🧩
Why NIR divides by 10
CBR and CDR are both “per 1 000”. Subtract them and you still have a number per 1 000. To turn per 1 000 into a percentage (per 100) you divide by 10. So 15 per 1 000 becomes 1.5%.
How the global population has changed
For almost all of human history the population barely moved. From 10 000 BCE to 1700 CE the average growth rate was only about 0.04% per year. Then, from the middle of the 18th century, growth turned exponential.
In 1800 the world population was about 1 billion.
By 2024 it had passed 8 billion.
Most of that is down to improvements in medicine, agriculture and technology, which cut death rates long before birth rates followed.
The growth rate peaked at roughly 2.1% in the late 1960s and has been falling ever since.
This is the single most misread graph in the topic. Slower growth does not mean fewer people, it means the population is climbing less steeply.
Read the two lines separately. The amber line is the number of people. The blue line is how fast that number is changing. The blue line has been falling since the late 1960s while the amber line has more than doubled. Both statements are true at the same time.
Predicting what happens next
Population models are used to predict future growth. They take into account birth rates, death rates, fertility rates and migration, and they help policymakers make decisions about resource use, healthcare and urban planning.
The United Nations publishes three scenarios rather than one number, because the future depends almost entirely on what happens to fertility.
The three lines start from the same point. What separates them is only how many children the average woman has.
UN scenario
What it assumes
Where it leads
High fertility
Higher birth rates continue
A more rapid population increase, pushing past 11 billion
Medium fertility
A steady decline in fertility rates — the most likely scenario
Moderate growth to around 9.7 billion by 2050 and roughly 10 billion by 2100
Low fertility
Fertility rates drop significantly
Slower growth, then a shrinking population
Different textbooks quote different figures for 2100 because the UN revises its projections every couple of years. Do not panic about the exact number. What examiners want is that you know there are three scenarios, that the middle one is most likely, and that the difference between them comes from fertility.
Predicting fertility is genuinely hard, which is where the uncertainty in these forecasts comes from. Changes in cultural norms, economic conditions and government policies can all shift fertility rates in ways nobody sees coming.
Where the growth is happening now
Countries that went through Industrial Revolutions in the 18th and 19th centuries had rapid population growth at the time.
Today those same countries are developed, and their growth rates have fallen. In some cases they have fallen so far that the total population is in decline — Japan is the standard example.
The fastest growth today is in developing countries that are rapidly industrialising, particularly in sub-Saharan Africa.
The pattern repeats. Rich countries are not different from poor ones. They are further along the same path. Every country that has industrialised has seen death rates fall first, then birth rates, and growth slow afterwards. You will meet this idea again as the demographic transition model.
Worked examples
WE 1
Crude birth rate and crude death rate
A country has 25 000 live births in a year and a total population of 500 000. In the same year a second country records 15 000 deaths from a population of 750 000. Calculate the CBR of the first and the CDR of the second. (2 marks)
Step 1: crude birth rate
CBR = (25 000 ÷ 500 000) × 1 000 = 0.05 × 1 000
Step 2: crude death rate
CDR = (15 000 ÷ 750 000) × 1 000 = 0.02 × 1 000
CBR = 50 births per 1 000; CDR = 20 deaths per 1 000always write the unit — “per 1 000 people per year”, not just “50”
WE 2
Total fertility rate
A country has these fertility rates per 1 000 women: 15–19 years, 20; 20–24 years, 85; 25–29 years, 100; 30–34 years, 80; 35–39 years, 40; 40–44 years, 10; 45–49 years, 2. Calculate the total fertility rate. (3 marks)
Step 1: add the age-specific rates
20 + 85 + 100 + 80 + 40 + 10 + 2 = 337 births per 1 000 women
Step 2: multiply by 5
337 × 5 = 1 685 births per 1 000 women (each band covers five years)
Step 3: convert to children per woman
1 685 ÷ 1 000 = 1.685
TFR = 1.69 children per woman (2 d.p.)this is below replacement level, so this population would shrink without migration
WE 3
Natural increase and doubling time
A country has a CBR of 25 births per 1 000 and a CDR of 10 deaths per 1 000. Calculate the natural increase rate, and then the doubling time. (3 marks)
Step 1: natural increase rate
NIR = (CBR − CDR) ÷ 10 = (25 − 10) ÷ 10 = 1.5%Step 2: doubling time using the rule of 70
DT = 70 ÷ growth rate = 70 ÷ 1.5
NIR = 1.5% per year; DT = 46.7 yearsthe rule of 70 assumes the growth rate stays constant, which it rarely does — say so if asked to evaluate
💡 Exam tips
Be able to define crude birth rate, fertility rate, life expectancy and doubling time. Definitions come up every year.
Learn the four flows: births and immigration in, deaths and emigration out.
Show your working in calculations. Method marks are available even if the final number is wrong.
Give units with every answer: per 1 000 people, children per woman, years, per cent.
Know the milestones: 1 billion in 1800, over 8 billion in 2024, growth rate peaking near 2.1% in the late 1960s.
Remember the three UN scenarios and that the medium one is treated as most likely.
⚠ Common mistakes
Confusing growth rate with population size. The growth rate has been falling for decades while the population has kept rising.
Forgetting to multiply by 1 000 in CBR and CDR, which leaves you with a tiny decimal.
Forgetting the × 5 in TFR. Each age band is five years wide.
Mixing up NIR and total population change. NIR ignores migration.
Treating the rule of 70 as exact. It only holds if the growth rate stays the same.
Saying fertility rate and birth rate are the same. CBR is per 1 000 people; TFR is children per woman.
Up next: Managing Population Growth. You can now measure a population. The next question is what governments do when they decide those numbers are going the wrong way.
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