Source: Office for National Statistics, Mid-year population estimates and National Population Projections 2010-based; Government Actuary’s Department, Past national projections database
The Impact of Migration on UK Population Growth
Based on official population estimates and population projections, this briefing examines the impact of migration on recent and future UK demographic trends.
- Half of the increase of the UK population between 1991 and 2010 (2.4 million in absolute terms) was due to the direct contribution of net migration.
- As a result of different net migration assumptions, the current ONS population projections produce a range of variation of nearly 4 million in the projected size of the UK population in 2035 (between 71 and 75 million).
- In the principal projection the cumulative net inflow of post-2010 migrants accounts for 47% of total population growth. A further 21% of projected population growth is attributable to the additional contribution of new migrants to natural change (i.e. births and deaths).
- The projected contribution of net migration to population change considerably differs across the four UK constituent nations. Without net immigration Scotland’s population would stagnate over the next two decades and decrease in the longer term.
- Net migration assumptions have been continually revised in the projections released since the mid-1990s, reflecting rising levels of net migration and the high uncertainty of migration forecasting. Largely as a result of these revisions, the projected population size in 2031 is currently 11 million higher than in the 1994-based projections.
Understanding the evidence
In the UK statistical system long-term international migrants are defined as people who move into and out of the country for at least 12 months. Net migration is the balance between immigration and emigration over a given time period. In demographic terms, natural change – i.e. the difference between the number of births and deaths – measures the contribution of vital events to the dynamics of the population. Immigration and emigration contribute to population change not only by altering the number of individuals in the country at a given time (direct contribution) but also by affecting natural change (indirect contribution).
The Office for National Statistics (ONS) produces annual estimates of the resident population of England and Wales and estimates for the UK as a whole by collating data provided by the Northern Ireland Statistics and Research Agency (NISRA) and by the General Register Office for Scotland (GROS) (ONS 2010). The population at 30 June of a given year (stock) is obtained by annually ‘updating’ the most recent census population count with data on demographic events contributing to population change between these two dates (births, deaths and migration flows). Population estimates made between census years are revised retrospectively so as to provide a consistent series of population estimates over time. Mid-year population estimates are also used as the base-year population of demographic projections.
Population projections are calculations showing the future development of a population based on a set of assumptions about fertility, mortality and net migration. Official UK projections are revised every two years by updating base-year population estimates and assumptions underlying future demographic dynamics so as to reflect the latest available information. Current projections take mid-2010 as the beginning of the projection period (ONS 2011). The projection outputs consist of one principal projection reflecting the most ‘likely’ population developments on the basis of recently observed trends, and 20 variant projections, intended to capture the uncertainty underlying the assumptions made by showing the impact on population dynamics if one or more components of demographic change differ from the principal projection. In the principal projection, net migration is assumed to level off at +200,000 per year from 2016 onward. The most important variant projection for the purposes of this briefing is the ‘zero net migration’ (also called ‘natural change’), which assumes migration inflows and outflows exactly equal at all ages throughout the projection period, with fertility and life expectancy at the same levels as the principal projection. In this scenario future population change is driven only by natural change. The comparison between the principal projection and the zero net migration variant projection shows the overall effect of net migration on population trends – i.e. including both the direct contribution and its impact on natural change. Two other variant projections illustrating the demographic impact of higher or lower net immigration – also assuming the same fertility and mortality as the principal projection – have also been produced. These resulted in a high migration variant (long-term annual net migration at +260,000) and a low migration variant (+140,000 per annum).
Population estimates show that net migration was a major component of population growth over the past two decades (Figure 1).
In particular, annual net migration has substantially increased since the beginning of the 1990s, exceeding natural change as a driver of UK demographic trends in all years between and including 1998 and 2007. However, natural change has remained positive throughout the last two decades and has also significantly increased from 2001 onwards, in particular as a result of a rise in the number of births. In 2007-08, 2008-09 and 2009-10 natural change contributed more to the growth of the UK population than net migration. Overall, between mid-1991 and mid-2010 natural change and net migration have contributed equally to UK population change – each resulting in an additional 2.4 million people.
However, this retrospective analysis does not account for the contribution of past migration to natural change – mainly to births, given that migrants are mostly young, healthy individuals. The number of births over a given period is determined both by the size and age structure of the female population and by fertility rates (i.e. the average number of children per woman in each age group). Migration impacts on both factors – i.e. it affects the number of women of childbearing age and, if migrant women have different fertility patterns, the total fertility rate of the population as a whole. Recent analysis has shown that the increase in the number of births observed between 2001 and 2007 was mainly driven by rising fertility levels among UK born women and by the increase in the number of foreign-born women of childbearing age. However, the decreasing size of the UK-born female population of reproductive age and changes in its age structure have brought about a reduction in the number of births, so that the overall contribution of foreign-born women to the increase in number of births has been estimated at 65% (Tromans et al. 2009: 33). While this figure certainly points to the significant indirect contribution that immigration is making to UK population trends, it has to be stressed that this analysis, by referring to country of birth: i) considers a temporally broad definition of the migrant population (i.e. the overall impact on births of in-migration over the past three or more decades, not only the contribution of those who moved to the UK during the observed period) and ii) does not single out the effect of emigration (of both UK- and foreign-born women) and of immigration of UK-born women.
Figure 2 shows the projected size of the UK population in the period to 2035, including high migration and low migration variants of the 2010-based projections.
In the principal projection, the size of the UK population is projected to increase by 11 million between 2010 and 2035 – from 62.3 million to 73.2 million, an increase of 18%. In this demographic scenario, the UK population will ‘hit’ 70 million in 2027 (i.e. two years earlier than in the 2008-based projections, see below). The different net migration levels assumed in the high migration and low migration variants (± 60,000 per year) lead to a variation after 25 years of ±1.9 million people – or, in relative terms, a difference of 5.4% between the low migration and high migration variant.
Figure 3, which refers to the 2010-based principal projection, shows the breakdown of the projected population increase into three components: the natural change that would occur in the absence of migration during the projection period (zero net migration variant); the direct contribution of post-2010 net migration to population change (i.e. the number of individuals who will migrate to the UK minus the number of those who will leave the country); and the indirect contribution of net migration to natural change.
The UK population is projected to rise both because of positive natural change and because of positive net migration. Population growth in the absence of further migration would total 3.5 million, equivalent to 32% of the total increase in the principal projection. However, the size of the UK population with no additional net migration would level off at 65.7 million over the next two decades and would eventually decline if the projections are carried forward beyond 2035. In the principal projection the cumulative net inflow of new migrants accounts for 47% of total population growth, i.e. an addition of 5.1 million. The additional (indirect) contribution of post-2010 immigrants to natural change until 2035 is estimated at 2.3 million, i.e. 21% of projected population growth. In total, therefore, over two-thirds of the expected increase in the UK population is attributable, directly or indirectly, to future net migration. It should also be emphasised that, while these calculations are based on the same assumptions about future fertility and mortality rates irrespective of the assumed level of net migration, fertility and mortality rates for recent migrants are likely to differ, to some extent, from those for the long-established population – e.g. assuming higher fertility rates for post-2010 immigrant women would lead to a larger indirect contribution of migration to natural change.
Figure 4 compares projected population growth rates in England, Wales, Scotland and Northern Ireland between 2010 and 2035, in the principal projection and in the zero net migration variant.
Demographic and migration trends differ considerably across the four UK constituent nations, and future population scenarios reflect these differences. In the principal projection, England has by far the largest population growth rate (19%), while Scotland has the slowest rate of demographic growth (10%). The overall (i.e. direct and indirect) contribution of net migration to population change also varies across UK nations. Net migration is the major component of projected demographic growth in England and Wales, accounting for 67% and 79% of the projected population growth respectively. In Scotland, net migration is the only factor underlying long-term population growth: without net immigration Scotland’s population would remain essentially stable for the next two decades and decrease over a longer time horizon. In contrast, in Northern Ireland natural change is projected to be the sole driver of future population trends.
Population projections are not forecasts, i.e. they do not attempt to predict the impact of changes in the political, economic, social and cultural realm which may affect demographic patterns and trends. They are in general purely mechanical calculations, formalising the implications of assumptions. Projections are normally reliable for the short to medium term but uncertainty increases the further they are carried forward in time. Any upward or downward changes in fertility, mortality and migration assumptions, compounded over time, can lead to significant variations in the projected population size and structure. Future international migration is more difficult to project than fertility and mortality because migration flows are typically affected by sudden changes in economic, social, or political factors which are hard to predict or quantify. Migration assumptions are therefore the major source of uncertainty for long-term population projections, particularly in demographic regimes such as the UK which are characterised by low fertility and low mortality levels. In addition, the use of net migration as a statistical construct to operationalise the combined effect of immigration and emigration could cause additional uncertainty because net migration is more volatile than ‘gross’ flows (Herm and Poulain 2010).
In order to reflect the information provided by the most recently observed demographic trends, assumptions of future levels of fertility, mortality and migration are continually updated in subsequent revisions of population projections. In particular, the high volatility of recent migration inflows and outflows has resulted in numerous and sizeable revisions of future net migration assumptions made in different sets of population projections released throughout the 1990s and 2000s. As shown in figure 5, in the 1994 projections net migration was assumed to return to zero in the long-term, reflecting the balance between immigration and emigration proximate to zero observed during the 1980s and early 1990s. As of the mid-1990s the size of the UK population was projected to peak at 61 million in 2023 and then start to decrease.
|Summary of assumptions|
|Projection base-year||Annual net migration (long-term)||Total Fertility Rate (long-term)||Life expectancy at birth (2031)|
In further sets of projections, assumed net migration levels have risen several times in response to the rapid increase in migration flows to and from the UK – and due to some improvement in survey coverage and procedural changes in the estimation of long-term flows. Changes in fertility and mortality assumptions have been comparatively less significant. As a result of these adjustments, projected population growth rates have also progressively increased: in the most recent projection (2010) the estimated size of the UK population in 2031 is 11 million higher than in projections produced in the early 1990s. Increases in net migration assumptions were the main drivers of the higher projected demographic growth rates until the 2004 revision and in the 2010 revision, while the combined effect of revised fertility and mortality assumptions has played a greater role in the faster projected population increase resulting in the 2006 and 2008 projections.
- Herm A. and M. Poulain. “International Migration Data as Input for Population Projections.” Working Paper 20, Joint Eurostat/UNECE Work Session on Demographic Projections, Lisbon, 28-30 April 2010.
- ONS. “Background and methodology: 2010-based national population projections.” ONS, London, 26 October 2011. http://www.ons.gov.uk/ons/dcp171776_232313.pdf.
- ONS “Mid-Year Population Estimates Short Methods Guide.” ONS, London, 2010. http://www.statistics.gov.uk/downloads/theme_population/short-methods-gu....
- Tromans N, E. Natamba, and J. Jefferies. “Have Women Born outside the UK Driven the Rise in UK Births since 2001?” Population Trends, 136 (2009):28-42.
- Coleman D. “Projections of the Ethnic Minority Populations of the United Kingdom 2006–2056.” Population and Development Review, 36(3) (2010): 441–486.
- Howe N. and R. Jackson. “Projecting Immigration. A Survey of the Current State of Practice and Theory.” Working Paper 2004-32, Center for Retirement Research at Boston College, Chestnut Hill, MA, 2005. http://csis.org/publication/projecting-immigration.
- Keilman N. “UK National Population Projections in Perspective: How Successful Compared to Those in Other European Countries?” Population Trends, 129, (2007):20-30.
- ONS “Births in England and Wales by Parents’ Country of Birth, 2009.” Statistical Bulletin, ONS, London, 2010c. http://www.statistics.gov.uk/pdfdir/births0810.pdf.
- Shaw C. “Fifty Years of United Kingdom National Population Projections: How Accurate Have They Been?” Population Trends, 128 (2007): 8-23.
With thanks to Phil Rees, Sarah Spencer and COMPAS colleagues of the Migration Observatory for their helpful comments on and earlier version of this briefing.