The experimental tool of social sciences

Microsimulation is a modelling tool applicable across all science and industry, whose importance grows with increasing computational powers and data availability. It simulates the mechanics of a large system at the level of its constituents, incorporating their individual characteristics, complex dynamics and mutual interactions. It can be used to analyse future scenarios, test and optimise strategies or design interventions. The technique challenges standard prediction methods by providing not only more accurate and precise answers, but also resolving a much wider range of possibly highly specific questions. For example, how much do we need to subsidise healthy food to reduce the T2 diabetes cost projected in 10 years by more than the expected sugar tax income? How to geographically allocate the support and target specific demographic groups over this time?

Averisera has built a fast, efficient and flexible microsimulation engine employing novel mathematical solutions and modern numerical techniques. Below we present some of its applications.

Demographic changes in the UK after Brexit

Presented at International Microsimulation Association World Congress, Turin 2017; published in International Journal of Microsimulation

The impending exit of Britain from the European Union (Brexit) following the referendum on 23 June 2016 continues to bring up questions about the future demographics of the country and their socioeconomic implications. The rich multiethnic, multicultural structure of the UK society has been historically shaped by several waves of economically motivated migration, the last of which began after the accession of Eastern European countries to the EU in 2004. Similarly, Brexit will influence its future composition through population shifts in migration patterns between Britain and the rest of EU. The country will undergo a significant demographic transformation, which can be expected to have a strong impact on the society and politics. Its exact nature will largely depend on the UK's stance on immigration after Brexit planned for the end of March 2019.

Microsimulation parameters

Initial population

Geographic regions

United Kingdom, European Union and non-EU countries
Individual characteristics

date of birth, age, sex, ethnicity (same as mother's), fertility, reproductive history, history and likelihood of migration (children below the age of 15 migrate with mothers), mortality
Simulation period

Data sources
Office for National Statistics: 1991, 2001 & 2011 UK Census, birth and death registrations, International Passenger Survey, Labour Force Survey

We use microsimulation to forecast and compare the impact of different policy scenarios on the UK demographics, depending on the referendum outcome and subsequent relationships with EU and non-EU countries:

  • Status quo extrapolates current population and migration trends
  • 2nd enlargement describes the UK remaining in the EU under unchanged conditions and the subsequent wave of immigration, similar in nature to the post-2004 wave from former Soviet Bloc, after the second EU enlargement in 2020
  • Soft Brexit assumes amicable parting between the UK and the EU, which does not change their relations significantly and thus the migration flows of the UK and EU citizens experience only slight changes
  • Hard Brexit drastically limits the migration between the UK and the EU as many migrants lose the right of residence or decide to return to their country of origin
  • Hello World compensates the drastically limited migration between the UK and the EU with the inflow of new immigrants from non-EU countries

We analyse the implications of the scenarios for the future age, sex and ethnic structure of the UK population, and estimate its basic socioeconomic indicators.


Click items in the legend of the graphs to display the difference between the chosen scenario and the Status quo case and the date axis to change its range.

The main drivers of the UK population growth are extending lifespan and immigration along with higher fertility rates of immigrant women. The latter accounts for differences between the considered scenarios, which become increasingly apparent after the Brexit date. The Brexit scenarios generally predict lower population size than the Remain ones. In particular, in the “2nd enlargement” case the population grows by 20% until 2061, achieving almost 70 million members, whereas after “Hard Brexit” it grows by only 5% to 61,5 million.

Click items in the legend to display the difference between the chosen scenario and the Status quo case and the date axis to change its range.

The chart shows the change in the median age between now and 2036 under the considered EU membership scenarios. We compare male, female and total UK population, and distinguish three (aggregated) ethnic groups: British citizens, EU immigrants and others.

The future median age of the UK population inevitably grows, exacerbated by Brexit. The trend concerns all sex and ethnic groups. The increase is least in Remain scenarios owing to the inflow of young immigrants from Eastern Europe and the new accession countries, as well as their children. In all Brexit scenarios, the median age is expected to rise sharply for the EU immigrant population, as its youngest members who have not been in the UK for long are more likely than their older peers to return to their country of origin taking their young children. The effect is more pronounced for females, who are a larger part of this group.

The percentage of women of reproductive age (15-45) in the UK population is expected to fall in all considered scenarios, which will negatively impact the natural growth. Brexit reduces their number sharply because of the specific age and sex structure of the EU immigrant group leaving Britain, namely younger and with a higher share of women than the UK average. The change is most prominent in the Hello World scenario, in which this group is replaced by the inflow of predominantly male immigrants from non-EU countries. The overall ageing of the British society imposes the same trend, although milder, on the Status quo scenario. It can be partially offset by the anticipated wave of immigration after the future EU enlargement, assuming its age and sex structure to be similar to the post-2004 Eastern European one.

The above effects cause the male to female ratio in the total population to increase in all considered scenarios.

Click items in the legend of the graphs to display the difference between the chosen scenario and the Status quo case and the date axis to change its range.

All scenarios reveal two periods in which the number of women of reproductive age and their share in the UK population increase: the opening decade of this century and around 2024. The first is caused by the 2004 EU enlargement and the ensuing flow of immigrants from Eastern Europe, while the latter by their children entering the reproductive age. They indicate past and forthcoming small “baby booms”.

Click items in the legend of the graphs to display the difference between the chosen scenario and the Status quo case and the date axis to change its range.

The dependency ratios, describing the relationship between dependents (children and/or elderly) and the working-age group (ages 15-64), are basic indicators of future economic and social stability. The graph shows their trends for the UK population under the considered scenarios. The dependency ratio experienced a large dip in the first decade of this century, caused by a simultaneous decline in fertility rates and the influx of A8 workers after the 2004 EU enlargement. The same effects explain the flattening of the old age dependency ratio (of elderly to working-age persons) in that period. After the influx subsided, and the female EU immigrants started having children, both curves picked up their previous upward trends driven by population ageing.

The curves fan out in 2019 highlighting differences between migration patterns in the analysed scenarios. Brexit yields highest dependency ratios owing to the outflow and restricted movement of the EU workers; increased numbers of non-EU immigrants cannot compensate this effect. The Remain scenarios slow down but do not stop the upward trend. All curves flatten after 2036, when “baby boomers” enter the workforce and the life expectancies stops to grow.

Click items in the legend of the graphs to display the difference between the chosen scenario and the Status quo case and the date axis to change its range.

The divergence between the considered scenarios is mainly driven by working-age EU immigrants. As shown in the bar charts below, from 2016 to 2036 their number doubles if the UK remains in the EU, grows slightly after “Soft Brexit”, and almost halves in the radical Brexit scenarios. The “Hello World” scenario envisages the arrival of new non-EU workers, the group growing in all scenarios owing to high fertility rates of their UK population.

Click the chart bars to display detailed ethnic composition of the immigrant group in the chosen scenario.

Population pyramids for all considered scenarios have the shape characteristic of an ageing society. In Remain scenarios their lower halves are broader owing to the influx of young EU immigrants. However, this broadening does not extend down to the lowest age groups, highlighting the difference between population change due to natural growth and immigration.


Ethnic composition of the population can change by natural growth and migration. Under Remain scenarios, the UK population grows mainly through immigration from the EU and new accession countries after the next enlargement. New immigrant women, who will arrive in the UK after 2016, contribute up to 50% of births to natural growth in this group. Under Brexit scenarios, British expats return to the country, but their relatively small number and predominantly post-reproductive age prevent counterbalancing the negative natural growth of their ethnic group. The native British share of the UK population shrinks contiunously also owing to high fertility rates of several other relatively numerous ethnic groups such as Pakistani or Africans. This above-average fertility rate of non-EU immigrants, as well as the Eastern European “baby boomers” are the main factors driving the population growth.

Population change between   and    in       


The differences between Brexit and Remain cases, although significant, are mostly those of degree. In the constantly growing population, the median age and dependency ratios will continue to rise, and the percentage of women of reproductive age will fall regardless of the EU membership status. These changes result mostly from the interplay between extending lifespans and lower average fertility rates. The influx of immigrants from current and prospective EU countries, as well as their children, could partially forestall them. This young group with the prevalence of women cannot be replaced with the boost of non-EU immigrants. At the same time, the share of native British in the UK population will shrink, even after their repatriation and the exodus of EU immigrants after the Brexit, owing to high fertility rates of other ethnic groups.

The above demographic changes will have important fiscal consequences both on the revenue (lower number of people of working age) and expenditure (higher medical care costs and a fraction of pensioners offset by lesser demand on school and maternity services) side. Under all considered scenarios, the UK demographics will undergo a significant transformation, which can be expected to have a strong impact on the society and politics. Its exact nature will depend on the UK's stance on immigration after Brexit. However, the presented results indicate that alleviating the growing strain placed by the ageing population on the country's social services will require additional interventions.

Technical details

The article “Microsimulations of demographic changes in England and Wales under different EU referendum scenarios” describes the details of our modelling approach.