IB Syllabus focus:
‘Births and immigration add people; deaths and emigration remove people. Crude rates (per 1,000) quantify these flows and can be applied from towns to global populations.’
Population size changes through inputs and outputs, where births and immigration add people, and deaths and emigration remove them. Understanding these flows enables comparisons across local, national, and global scales.
Population Inputs and Outputs
Population Inputs
Population inputs refer to processes that increase the number of people in a defined population. These include:
Births – the total number of live births within a given time period.
Immigration – the arrival of individuals into a population from other areas.
Both inputs expand the population, though they arise from different mechanisms: biological reproduction and migration patterns.
Birth Rate: The number of live births per 1,000 people in a population per year.
This measure allows for comparisons between different populations, regardless of their absolute size.
Population Outputs
Population outputs refer to processes that decrease the number of people in a defined population. These include:
Deaths – the number of people who die within a given time period.
Emigration – the movement of individuals out of a population to live elsewhere.
Death Rate: The number of deaths per 1,000 people in a population per year.
Like the birth rate, this figure provides a standardised measure across regions.
Measuring Population Change
Crude Rates
The most basic measurements are called crude rates, which quantify births and deaths per 1,000 individuals per year.
Global distribution of the crude birth rate (CBR), expressed as live births per 1,000 population per year. Choropleth shading enables quick comparison of input intensity across countries, matching the per-1,000 convention used in demographic rates. The map shows relative patterns only and does not adjust for age structure. Source.
Crude Birth Rate (CBR): The total number of live births per 1,000 people in a population per year.
Crude Death Rate (CDR): The total number of deaths per 1,000 people in a population per year.
These are termed “crude” because they do not account for the age and sex distribution of the population, which can significantly affect results.
Net Migration
Migration contributes both inputs (immigration) and outputs (emigration).
Net Migration: The difference between the number of immigrants and emigrants per 1,000 people in a population per year.
Positive net migration indicates more people entering than leaving, while negative values reflect the reverse.
Comparison of annual population growth when migration is included versus excluded. The “without migration” series isolates natural change (births – deaths), while the “with migration” series reflects the full balance including immigration – emigration. The figure is a time-series chart and therefore includes temporal detail beyond the syllabus definition. Source.
Population Balance
The balance of inputs and outputs determines whether a population grows, declines, or remains stable. Population growth occurs when births and immigration exceed deaths and emigration, while decline results from the opposite.
Population Change = (Births + Immigration) – (Deaths + Emigration)
This simple equation highlights how each component contributes to population dynamics.
Scales of Application
Local Populations
In towns and cities, local population change is influenced heavily by migration flows, such as rural–urban migration. Crude rates remain useful but must be understood in the context of short-term movements.
National Populations
At the national level, governments track crude birth and death rates along with migration data. National policies, economic conditions, and healthcare systems strongly influence these rates.
Global Populations
At the global scale, migration between countries balances out, leaving births and deaths as the sole inputs and outputs. Thus, global growth is shaped entirely by fertility and mortality.
Factors Influencing Inputs and Outputs
Cultural and Social Factors
Religion and tradition may encourage higher fertility.
Education, especially of women, tends to lower birth rates.
Economic Factors
Job availability may attract immigration.
Poverty and lack of opportunity may drive emigration.
Political and Environmental Factors
Conflict often leads to large-scale emigration.
Natural disasters can also trigger sudden population losses.
Why Crude Rates Matter
Crude rates provide:
Comparability across different regions.
Trend tracking over time.
Policy guidance for governments managing health, housing, and infrastructure.
Although crude, these measures are accessible and widely used, forming the foundation for more detailed demographic studies.
FAQ
Sudden changes may result from external shocks such as wars, epidemics, famines, or natural disasters. These events can sharply increase death rates or reduce birth rates.
Government interventions, such as new healthcare systems or family planning programmes, can also create rapid demographic shifts. Short-term fluctuations are often followed by stabilisation once the crisis or policy effect passes.
Crude rates measure births and deaths per 1,000 people but ignore population structure.
Refined measures, such as age-specific fertility rates or infant mortality rates, adjust for particular age groups, making them more accurate for identifying underlying demographic patterns. Crude rates remain useful for broad comparisons but lack detail.
Migration is less systematically recorded than births and deaths, which are usually logged in civil registries.
Challenges include:
Informal or undocumented migration.
Seasonal or temporary movement not fully captured in official data.
Political reluctance to publish sensitive migration figures.
These factors make net migration estimates less reliable than natural change measures.
At the global level, migration cancels out, so only births and deaths determine growth.
At local or national levels, immigration and emigration strongly shape population dynamics. For example, a city may grow rapidly due to rural–urban migration, even if natural increase is low. Thus, migration has far greater significance in smaller-scale population changes.
Raw numbers do not allow meaningful comparison between populations of different sizes.
Per-1,000 rates standardise the measurement, enabling comparisons across towns, nations, or global regions. This method also reveals relative intensity of births, deaths, and migration, rather than being influenced by absolute population totals.
Practice Questions
Question 1 (2 marks)
Define the term crude birth rate and explain why it is described as “crude.”
Mark scheme:
1 mark for stating that crude birth rate is the number of live births per 1,000 people in a population per year.
1 mark for explaining that it is “crude” because it does not take into account the age or sex distribution of the population.
Question 2 (5 marks)
Using examples, explain how migration can act as both an input and an output in determining population change.
Mark scheme:
1 mark for identifying immigration as an input that increases population size.
1 mark for identifying emigration as an output that decreases population size.
1 mark for giving a clear example of immigration (e.g., workers moving into a country for economic opportunities).
1 mark for giving a clear example of emigration (e.g., people leaving a country due to conflict or natural disaster).
1 mark for linking migration to overall population change (net migration determines whether it increases or decreases the total population).
