Thursday, 5 May 2016

Mayhew and Smith “An investigation into inequalities in adult lifespan”

The Introduction to this paper states that “the gap in life expectancy between the shortest and longest lived is widening for the first time since the late 1870s”. The widening is attributed to the fact that lifestyles are the main reason for the trend because “Men in lower (sic) socio-economic groups are the most lkely to make damaging life-style choices”
What are the data used to reach these conclusions? Wisely, the authors only start to consider inequlity from the age of 30. Infant mortality would be pretty hard to attribute to foolish choices of health behaviour. The measure used is an Inter-Percentile Range (or IPR) which compares the average age at death in the 5% of people who die youngest with that in the 10% of people who are oldest at the time of their deaths. This is done separately for men and women  To give examples from their Table 1: in 1879 the average age of men who lived longest was 85.6 years compared to 39.7 years in the 10% that lived the shortest time, while in 2010 it was 95.7 years for the longest lived 5% and 62.4 years in the shortest-lived 10%. This gives an “inequality gap” of 44.9 years in 1879 and 33.3 years in 2010.
The argument that “inequality” is widening is based on a comparison of the difference between the top 5% and bottom 10% between 1879 and 1939 (where it decreased by 7.7 years for men) and the same difference between 1950 and 2010 (during which time it decreased by only 1.2 years).
This is a very interesting use of the Mortality Database, a rich source. The first thing I noticed was the huge difference between the changes over time in the top 5% and those in the bottom 10%. From 84.6 years to 97.5 is a hefty one, but a lot less than the difference between 39.7 and 62.4. The longest lived 5% gained around 13 years while the 10% who died youngest gained around 22 years. The paper also contains equally interesting data for women.
This analysis complements to the well-known increase in health inequality that motivated the Black Report, the Acheson Report, the work of the Marmot review and similar work. We know that health inequality increased steadily from the 1930s to 2001. But what these reports mean by “inequality” is totally different to what Mayhew and Smith mean. They are talking purely about the average age at death in groups defined according to whether the members were among the shortest or longest lived at any given period. The Black report and its successors were talking about groups based on social class. Recent reports from the Marmot Review group define groups in terms of the level of deprivation in a given residential area. In Mayhew and Smith’s analysis we have no idea about the income working conditions, residential conditions or occupation of anyone. In fact the idea that their analysis says something about health inequality requires us to assume already that a longer life has something to do with income or other measures of socio-economic position.

Similar work was done in the 1980s using a Gini coefficient for age-at-death. This work as far as I remember did not exclude deaths in childhood. You can think of the Gini coefficient as a kind of variance around the mean age at death. In the 19th century and early 20th, many more infants and children died, giving a far wider range of ages at which lots of deaths occurred. So no surprisingly the variance in age-at-death fell sharply over the 20th century. The authors of these studies (forgotten who they were ) argued (unlike Mayhew and Smith) that this was inconsistent with studies showing increasing health inequality. What makes the 2 types of study similar is that in both cases we actually have no idea about the socio-economic conditions of the people who were living longer or shorter lives. So in neither case is it possible to attribute any social cause to the demographic changes. 

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