UK Population Trends and the Demographic Transition Model

A growing population with a changing structure

Population change involves more than the total number of people. Population structure describes who makes up that population, especially their ages and sex. A country can become larger while also having a greater proportion of older people.

During the period since 2001, the UK experienced both population growth and changes in its age structure. Its estimated population increased from 60.4 million in 2005 to 65.1 million in 2015: a rise of about 4.7 million people over ten years. These are historical estimates, not present-day population figures.

Over the same period, the share aged 65 and over increased from 15.9% to 17.8%, while the share aged 0–15 fell from 19.3% to 18.8%. A falling percentage of children does not necessarily mean fewer children: the percentage is a share of a changing total population.

Separating natural change from migration

Two main processes change a country's population. Natural change is births minus deaths. When births exceed deaths, natural increase adds to the population. Net migration is immigration minus emigration: arrivals from abroad minus departures to live abroad.

$$\text{Population change} = (\text{births} - \text{deaths}) + (\text{immigration} - \text{emigration})$$

The UK experienced natural increase throughout 2001–2015. Births reached 813,000 in 2012. However, natural increase was not the only explanation for population growth: positive net migration also made a substantial contribution.

From 2004 to 2015, the direct contribution of net migration averaged more than 250,000 people per year. This was about 50,000 more people per year than the contribution from natural change over the same period. Migration was therefore a larger direct contributor to growth than natural change during these years.

In 2015, approximately 631,500 people immigrated to the UK and 299,200 emigrated. Subtracting departures from arrivals gives positive net migration of approximately 332,300 people. Counting arrivals alone would overstate migration's contribution to population growth.

Migration also has indirect effects. People who move to the UK may subsequently have children there, influencing future births as well as immediately adding to the population. During this historical period, rises in immigration often coincided with EU expansion, which enabled more people to move freely to the UK. For example, Poland and seven other central and eastern European countries joined the EU in May 2004, alongside Cyprus and Malta.

Reading a population pyramid

A population pyramid represents age and sex together. Age groups run vertically, with younger people at the bottom and older people at the top. Horizontal bar widths show how many people, or what percentage of the population, belong to each age group. Males are usually shown on the left and females on the right.

Read the axes and units before interpreting the shape. A wide bar means a relatively large age group; a narrow bar means a smaller one. Compare the younger, middle and older groups, then look for distinctive bulges or narrow sections. These features can reflect past changes in births, survival and migration.

The UK's 2015 age structure contained several recognisable birth cohorts. People in their mid-40s to mid-50s were numerous because they were born during the 1960s baby boom. A relatively large group aged 68 reflected the spike in births following the end of the Second World War. Smaller groups in their late 30s and early teens reflected lower fertility in the 1970s and early 2000s respectively. A pyramid therefore preserves traces of earlier population events, rather than showing only current conditions.

Comparing pyramids across time

A cohort moves upwards through a population pyramid as it ages. To compare 2005 with 2015, follow people into an age group ten years older rather than simply comparing the same age label.

For example, people aged 10–25 in 2005 were aged 20–35 in 2015. This cohort became larger over those ten years. Ageing moves people between age groups, but cannot add people to a cohort; deaths would reduce it. Its growth therefore indicates immigration into that cohort.

The male and female age structures were broadly similar in 2005 and 2015, but women outnumbered men at older ages, particularly above 80. Alongside the growing share aged 65 and over, this demonstrates how population growth and changing structure can occur together.

Connecting population evidence to demographic transition

The Demographic Transition Model (DTM) describes how birth and death rates tend to change as a country develops. Both rates are measured per 1,000 people per year, allowing populations of different sizes to be compared. When the birth rate is higher than the death rate, there is natural increase; a wider gap means faster natural growth.

The model moves from high birth and death rates in Stage 1 towards low rates in Stage 4. Death rates fall first as health and living conditions improve, creating rapid natural increase in Stage 2. Birth rates then fall in Stage 3, so natural growth slows.

The UK is conventionally classified as Stage 4: low fluctuating. It moved into this stage around the middle of the twentieth century. Both birth and death rates are low and relatively close together, so natural increase is limited compared with the rapidly expanding stages. Good healthcare helps keep death rates low, while later parenthood and smaller families help explain low birth rates.

The population evidence for 2001–2015 fits this interpretation. Births exceeded deaths, but migration contributed more to growth than natural change during 2004–2015. The ageing structure and relatively small younger age groups are consistent with a late-stage population, although a pyramid alone cannot establish its birth and death rates.

Why growth does not necessarily mean an earlier stage

The DTM does not include migration. A Stage 4 country can therefore have substantial total population growth even when the gap between births and deaths is small. The UK's positive net migration is essential to explaining this apparent contrast.

In the five-stage version of the model, Stage 5 involves births falling below deaths, producing natural decrease. An ageing population alone is not enough to place a country in Stage 5, and low fertility does not automatically mean deaths already exceed births. The UK's natural increase throughout 2001–2015 supports Stage 4 for that period rather than Stage 5.

The DTM is a simplified model, not a precise timetable. Baby booms, wars, economic changes and migration can produce fluctuations that its smooth stages do not capture. Use it to interpret the UK's population evidence, rather than expecting every change to follow the model exactly.