Jun Fujita, "National Guardsmen Questioning African American, 1919,"
1919, Photograph, Chicago Historical Society Archives. Chicago.
The paper's University of Chicago press release described Black et. al's results in the following terms:
UChicago scholars were part of a study that found if an African American man lived to age 65, his chances of reaching age 70 were 82.5 percent, if he remained in the South. But if he migrated, those chances shrank to 75 percent. For an African American woman who lived to age 65, the likelihood that she would live to 70 in the South were 90 percent but dropped to 85 percent if she migrated.
“Migrants were a self-selected group: Many of them had been preparing all their lives to make the move, and everything suggested they’d be healthier than their counterparts in the South,” said Dan Black, deputy dean and professor at the University of Chicago’s Harris School of Public Policy Studies, who co-authored the study. “But it was shocking to learn that, in terms of longevity, the migrants weren’t better off at all.”
Medical records indicate that migrants died at much higher rates from cirrhosis and pulmonary illnesses, which are closely linked to smoking and drinking—bad habits common among city dwellers at the time.
Black said there were other factors as well: chemicals and pollution from factories, higher population density (and, therefore, more contagious diseases), the shock of cold weather, and the stress from discriminatory housing markets and uneven employment prospects. 
The entire paper is worth skimming through.  Dr. Black and his team discuss the problem of self-selection quite thoroughly and prove to my satisfaction that the lower life expectancy in the north cannot be attributed to self-selection effects. They also demonstrate to my satisfaction that the black migrants who went north were better educated and more wealthy than the population from which they came. This makes the life expectancy numbers even more surprising--usually we think of all three variables as part of a single package.
It will take a bit of leg work to figure out exactly what caused African Americans living in the north to die sooner. Black et. al's suggestion that smoking and alcohol were the root cause is plausible, though more evidence backing this claim up is needed before we call the case closed. I'll let the biological anthropologists and public health specialists sort that out. I bring up the study here because it neatly illustrates a principle germane to the macro-historical debates we regularly have here at the Stage. Namely, "living standards" and wealth are not the same thing.
This topic has been on my mind since I finished Kenneth Pomeranz's The Great Divergence: China, Europe, and the Making of the Modern World last month. I was pleasantly surprised to find that it was not nearly as bad as its most severe critics make it out to be. Unfortunately for Dr. Pomeranz, that is a low bar to clear. I agree wholeheartedly with the critics who charge The Great Divergence with serious conceptual and methodological flaws.  I was a bit surprised, however, to find a conceptual flaw that none of the big name critics have--to my knowledge--already covered. The problem is found in the book's second section, where Pomeranz marshals a fascinating catalog of statistics to compare the living standards of Western Europeans and the Chinese of the Yangtze and Pearl River deltas on the eve of the industrial revolution. The data points he list range from infant fatality rates to protein intake to luxury good consumption. For almost every variable the story is the same: the Europeans were no better off (and sometimes worse off) than their Chinese contemporaries. Therefore, Pomeranz implies, during the 18th century the average European could not have been much wealthier than the average Chinese. 
This section of the book is built upon the false premise that wealth and the things usually included in standard of living indexes are functional equivalents that more or less move in tandem.  The trouble is that this is often not true. Research done since The Great Divergence was published proves the point. Folks like Bozhong Li, economic historian at Hong Kong University of Science and technology, and Jan Luiten van Zanden, historian at Utrech University, have put in the hard work needed to create a more exact measure of proto-Industrial household wealth in Western Europe and China, and have found that families in the Netherlands and Southern England were more than twice as wealthy as those in the Yangtze river delta.  As was the case with America's great migration, wealth and living standards simply did not match up.
It is not too difficult to find similar examples across human history. Perhaps the most compelling measurement of living standards are biometric measures of health like height. To simplify matters somewhat, the healthier a population is, the taller it becomes. Historians and anthropologists can thus track the health and physical well being of any community by tracking how heights have changed over time--say by examining skeleton remains or poring through conscription records. It is through this type of research we know that for the last century humankind has been growing taller. In this case wealth and health have followed each other quite closely. But we have also discovered lots of examples were this clearly was not so: paleolithic hunter-gathers were taller than wealthier neolithic farmers , Western Europeans jumped several inches after the Roman Empire fell (its wealth fell with it) , and most famously of all, heights in England and America fell after the Industrial Revolution began! .
|Average English soldier heights (in cm), 1730-1850 AD.|
Figure 2 in John Komlos, "Shrinking in a Growing Economy? The Mystery of Physical Stature during the Industrial Revolution". Journal of Economic History 58, no. 3 (1988), 781.
There is a temptation to look at these figures and declare that if a word like "wealth" does not include things like health, literacy, or high quality of life then it is not a meaningful concept at all. I have some sympathy with this view, but I think it mistakes what the physical basis for wealth really is: energy use. I have made this case before and so will not pursue the theme with great detail here except to note that over the grand course of human history energy consumption and economic growth have always been linked and (barring some epochal transition in the mode of production) they always shall be. This is because sustained economic growth occurs when people are able to make more stuff and do more things with less effort--in other words, with less energy. Better diets, disease incidence, or literacy rates are often downstream effects of rising productivity but there is nothing about productivity growth itself that must lead to any of these things.
 Dan Black, Seth Sanders, Evan Taylor, and Lowell Taylor, "The Impact of the Great Migration on Mortality of African Americans: Evidence from the Deep South," American Economic Review vol 105, iss. 2 (2015).
 An able review of this burgeoning debate is Martin Hewson, “Multi-Cultural vs. Post-Multicultural World History,” Cliodynamics: A Journal of Mathematical and Theoretical History, 3, iss. 2 (2012), 1-19.My favorite critique is Peer Vries, "The California School and Beyond: How to Study the Great Divergence?", History Compass, 8 (2010), 739-745.
 Kenneth Pomeranz, The Great Divergence: China, Europe, and the Making of the Modern World (Princeton: Princeton University Press, 2001), 30-42.
 The matter is complicated by the fact that most measures of standard of living--such as the HDI--include GDP per capita as one of its components. The fact that such an index was deemed necessary, however, ia itself evidence that wealth and "development" cannot be used interchangeably!
 Bozhong Li and Jan Luiten van Zanden, "Before the Great Divergence? Comparing the Yangzi Delta and the Netherlands at the Beginning of the Nineteenth Century," The Journal of Economic History 72, iss 04 (2012), 956-989.
 Amanda Mummert, Emily Esche, Joshua Robinson, George J. Armelagos, "Stature and robusticity during the agricultural transition: Evidence from the bioarchaeological record," Economics and Human Biology 9 (2011), 284–301.
 Koepke, Nikola and Joerg Baten. “The Biological Standard of Living in Europe During the Last Two Millennia,” European Review of Economic History 9 (2005), 25. Incidentally, Roman heights were taller earlier in the empire's history. See Geoffrey Kron, "Anthropometry, Physical Anthropology, and the Reconstruction of Ancient Health, Nutrition, and Living Standards" , Historia: Zeitschrift für Alte Geschichte, vol 58, iss 1 (2005), pp. 68-83.
 John Komlos, "Shrinking in a Growing Economy? The Mystery of Physical Stature during the Industrial Revolution". Journal of Economic History 58, no. 3 (1988): 779-802