Title: Understanding Recent Fertility Trends in the UK and Improving Methodologies for Fertility Forecasting
Funder: Economic and Social Research Council

Abstract: The aims of this research project are to examine the significant fluctuations in fertility levels in the UK in the last two decades, to investigate their causes, and to develop improved methodologies for fertility forecasting. Fluctuations in fertility levels have a significant effect on the population age composition; the post-war baby boom and the subsequent fertility decline are the main causes of the current population ageing in the UK. The need to understand and forecast fertility trends in the UK has never been more pressing. In the context of rapid population ageing and fluctuating migration numbers, there is an urgent need to measure the contribution of births to the growth and, even more importantly, age composition of the British population. A decline in fertility levels by 10% will lead to about 40 thousand fewer newborn babies in the UK annually. The task of predicting even short-term trends in fertility presents a major challenge for social scientists, the Office for National Statistics and government departments. With the support of the national statistical offices and a network of expert analysts and stakeholders, we aim to produce definitive, detailed measures of fertility changes in recent years in the UK. Using standardisation techniques we will decompose the overall changes into those attributable to compositional changes in the UK population, e.g. by country of birth and education, and those which are attributable to behavioural changes over time. We use these insights to develop new methodologies for more accurate forecasting of period fertility applying them to the UK and its constituent countries. The developed methodologies could be applied to project fertility in other industrialised countries. Using high-quality large-scale longitudinal data the project will, first, investigate childbearing trends by birth order in the UK in the last two decades to determine whether the increase and subsequent decline in the Total Fertility Rate (TFR) is attributable to the changes in first, second, third or higher-order births, or all of them. We also investigate whether differences in the TFR between the UK's constituent countries are the result of differences in the timing of childbearing, variations in levels of childlessness, or different family sizes among mothers. Second, the project will examine how much of the recent fluctuations in period fertility can be explained by the changes in fertility timing (i.e. women having children later or earlier in their lives), changes in population composition (e.g. increasing number of immigrants and their descendants), or changes in fertility behaviour (i.e. women have fewer or more children). Third, we will investigate whether certain sub-populations (e.g. educational groups) have changed their fertility behaviour more than others. We will assess whether the empirical evidence is consistent with theoretical expectations on the role that wider social changes, including the family policies introduced in the UK in the late 1990s and economic recession, play in changing fertility levels in the first decades of the 21st century. Finally, the project will propose a novel method to forecast future fertility in the UK. The forecast will draw on the analysis of fertility trends by birth order and will include uncertainty in fertility projections.

Lay abstract: The last two decades have witnessed dramatic fluctuations in fertility levels, which were not predicted by demographers or government statisticians: Fertility significantly increased in the first decade of the 21st century, whereas it has declined thereafter. These fluctuations have significant implications for planning and policy making, at both national and local levels. For example, the fertility increase between 2001 and 2012 led to more than 60 thousand additional births in the UK annually. The causes of the recent fertility dynamics are unclear. Some researchers attribute the recent fluctuations in fertility levels to changes in fertility timing - i.e. the postponement or acceleration of childbearing. Others emphasise the importance of changes in population composition or changes in childbearing behaviour in response to past policy changes and the post-2008 economic recession. Birth registration data used by government statisticians at the Office for National Statistics (ONS), National Records of Scotland (NRS) and Northern Ireland Statistical Research Agency (NISRA) inform us about the total number of births and aggregated fertility measures; however, they do not provide information about childbearing trends by parity (birth order), which is critical to understanding and predicting fertility trends. High-quality large-scale longitudinal data provide the opportunity to conduct a detailed analysis of parity-specific fertility; for example, to determine whether fertility has recently declined because of the (further) postponement of childbearing and increased childlessness among women or because of declining family size among mothers (e.g. fewer third births). Childbearing is naturally a sequential process; decisions on having an additional child are likely to be evaluated on the basis of experience with previous children. Detailed analysis of fertility by parity will thus significantly enhance our ability to forecast future fertility. In this project we will harmonise census-linked administrative data from the ONS Longitudinal Study, Scottish Longitudinal Study, and Northern Ireland Longitudinal Study, together with survey data from the Understanding Society study. The project is thus novel in that it uses data from the all four UK constituent countries; focuses on the analysis of childbearing trends by birth order, and brings together experts in demography and statistical forecasting to develop better methods for fertility forecasting. First, we will calculate annual parity-specific fertility rates by UK country to determine how much changes in fertility levels are attributable to the changes in first, second, third or fourth births. We will then adjust fertility rates for characteristics of that population (e.g. place of birth, educational level) to determine how much a change in fertility levels in the UK over time is attributable to changes in population composition, and how much to changes in childbearing behaviour, possibly as a result of changing policies and economic environment. Finally, we will use information on parity-specific fertility to forecast future fertility levels in the UK using Bayesian methodology. The project will bring together researchers from the Universities of St Andrews and Southampton, as well as government statisticians from ONS, NRS, and NISRA, to work on an important policy-relevant topic. The project will greatly improve our understanding of the factors associated with changing fertility dynamics in the UK and will show how existing large-scale longitudinal datasets can be used for cross-country analysis of fertility by birth order. It will also significantly improve the methodology used for fertility forecasts for the UK and its constituent countries. A better understanding of the present childbearing trends and forecast of the future developments will be critical to inform the planning of demand for various public services (e.g., nurseries, school places and housing).

Research activity codes: 1.2 PSYCHOLOGICAL AND SOCIOECONOMIC PROCESSES;1.4 METHODOLOGIES AND MEASUREMENTS
Health categories: REPRODUCTIVE HEALTH AND CHILDBIRTH