One of the stylised facts of the UK is that there's a large health and mortality gap. Michael Marmot and others continually tell us that this is because of social and economic inequality. We keep trying to point out that this is not quite so. For people are never measuring either the health nor lifespan of where people come from. Rather, mortality and lifespan are measured in the place where people drop dead. This has an obvious implication: given that people do migrate we are to some extent seeing people self-selecting into poor and richer areas. And we know very well that richer people live longer.

Interestingly, there's new evidence coming out about Glasgow. The poorer areas of that city are continually held up as evidence of Marmot's contention: economic inequality leads to that health inequality. However, this newer point rather supports our contention:

The mystery of Glasgow’s “sick man of Europe” status started to rear its head more than half a century ago. But now, for the first time, researchers from the Glasgow Centre for Population Health (GCPH) claim to have found hard evidence of a number of key factors that explain it.

In a new report, History, politics and vulnerability: explaining excess mortality, they claim a combination of the historic effects of overcrowding, poor city planning decisions throughout the 1960s, 70s and 80s and a democratic deficit – or lack of ability to control decisions that affect their lives – are among reasons why Glaswegians are vulnerable to premature death.

The research has been endorsed by some heavy hitters including Sir Harry Burns, formerly the chief medical officer for Scotland, Tom Devine, professor of history at Edinburgh University, and Oxford University geography professor Danny Dorling. But the findings are not about eating fewer chips and stopping smoking; they are deeply political.

According to Chik Collins, co-author of the report and professor of applied social sciences at the University of the West of Scotland, new research about “skimming the cream” of the city’s population to rehouse its “best” citizens in new towns, is particularly striking.

The research based on Scottish Office documents released under the 30-year rule shows new towns such as Cumbernauld, East Kilbride and Irvine were populated by Glasgow’s skilled workforce and young families, while the city was left with “the old, the very poor and the almost unemployable”.

One way to read this (and we do not insist that this is all of it but do insist that this is some of it) the mortality rates and lifespans of that total population haven't changed at all: or more likely have increased along with more general rates across the population. But that section of the population which always did have those lower lifespans has been left behind in those poorer areas and that which always had the longer have moved out.

This is much the same as noting that Bournemouth (and the archetype, Frinton On Sea) has a longer average lifespan than much of the country. But these are retirement towns, where many people only move there upon retirement. Those who are still alive at retirement age and also have the financial resources to move at that age do indeed have a longer lifespan than the population average.

We are equally certain that this explains at least some of the Case and Deaton finding about lifespans in America. Poor rural whites appear to have falling average lifespans, especially in Appalachia. Other studies of these areas point out that anyone who manages to get into college from these areas does so and then doesn't come back. The population is self-sorting into different geographic areas. With those staying being those who always did have the lower lifespans: but that's what we're now measuring in those areas. Andrew Gelman, who is investigating these numbers in detail tells us that this theory is not wrong but has not yet been shown to be right (or, if you prefer, has not yet been shown to be wrong).

We really are pretty sure that this is at least part of the explanation, as with Glasgow. Those who always did have longer average lifespans have left the area: the average lifespan of those remaining is no lower than it ever was, it just looks that way as we're now only measuring a subset of the original population.