Unravelling sex ratios, what can they tell us about census, population estimates and recent trends in migration?

Steve Smallwood, Office for National Statistics, UK
Sofie De Broe, Office for National Statistics, UK

This paper aims to understand the sex ratio observed in England and Wales (EW) population estimates and the extent to which its pattern could reflect recent trends in migration. In particular it looks at the changes in sex ratio patterns resulting from the population estimates derived from the 2001 Census and aims to identify plausible explanations for its subsequent trend. In most countries, low sex ratios at younger working ages are observed in census data, which leads to questions about their plausibility. One possibility may be that geographically mobile young males are more likely to get undercounted relative to their female counterparts. Even after using statistical techniques to adjust for under enumeration a sharp drop in the sex ratio was observed in the EW Census of 2001 around age 18 and similarly appears in the mid-2001 population estimates based on the Census. A noticeable feature of population estimates for subsequent years is that the drop observed around 2001 is ageing forward. In this paper, the authors will present some of the evidence in order to help explain the sex ratio pattern in the 2001 population estimates and subsequent years. The authors use the 1981, 1991 and 2001 Census data and compare these with a ‘synthetic’ population of those born in EW estimated to be alive today, created by applying historical mortality rates available from the Government Actuary’s Department to births, excluding the effect of migration. Patterns in sex ratios among cohort stocks of EW and non-EW born migrants are presented. Additionally, migration data from other sources are used to examine the sex ratio of migrant stocks and flows. The plausibility of the developing sex ratio is considered as well as whether the sex ratio patterns between 2001 and 2006 tell us something about recent trends in migration.