In 2015, Anne Case and Angus Deaton (2015) made headlines for documenting that the mortality rate of non-Hispanic white Americans in midlife had been rising since the beginning of the 21st century. This finding was so surprising because it cut against the trajectory of mortality in the US in the previous century. As Deaton (2013) had shown, Americans made tremendous gains in life expectancy over the 20th century, due in large part to the decline of deaths from infectious disease. Life expectancy in the US rose by more than 20 years between 1900 and 1950 (Carter et al. 2006).
The decline in deaths from infectious disease in the first half of the 20th century is well known and well documented (Armstrong et al. 1999, Haines 2001, Cutler et al. 2006), but there is surprisingly little evidence about whether it took place uniformly across the regions of the US. Understanding regional variation in the infectious mortality decline is important because scholars still debate the decline’s causes (e.g. Cutler and Miller 2005, Anderson et al. 2018).
The regions of the US varied in the compositions and densities of their populations, their climates and disease environments, and the extent of their public health interventions. Observing variation in infectious mortality over space gives researchers a warrant to search for causes of the decline that may vary from city to city and region to region.
To study regional differences in the decline in deaths from infectious diseases, we digitised 49 years of data on deaths by cause for all reporting US cities and classified causes of death as infectious or not (Feigenbaum et al. 2019). Because deaths from infectious disease are almost always highest among the youngest and oldest age groups, we standardised our infectious disease death rates by age to ensure that the patterns we observe are not driven by the fact that some cities have more very young and very old people. We focused on cities to make certain that we compared like with like. Other units, like counties and states, vary in their degree of urbanization, and rates of death are very different in urban and rural areas. For instance, in 1900, mortality rates were higher in cities than in rural areas, but by mid-century, the opposite was true (Haines 2001).
In most of the figures below, we plot cities' logged comparative mortality ratios. A city's comparative mortality ratio is the ratio of its actual mortality to its expected mortality, given its age distribution and a standardised mortality schedule. For instance, an unlogged comparative mortality ratio of 1.5 reflects infectious mortality 50% above what we would expect given a city's age distribution and a standardised mortality schedule. For simplicity, we call the logged comparative mortality ratio ‘infectious mortality’”,
In Figure 1, we plot median infectious mortality in the four census regions. Panel 1a shows the raw infectious mortality rate per 100,000 people—before the data were age-standardised. The most notable feature of the plot is that the urban South's infectious mortality rate exceeded that of the other regions for the entire period.
In panel 1b, we age-standardise the data to check that our results do not simply reflect differences in cities' age distributions. Here again, we observe much higher infectious mortality in the urban South than in cities in the other regions.
Finally, in panel 1c, we restrict our sample to cities for which we have data in every year from 1900 to 1948. The results remain unchanged: southern urban infectious mortality continues to be higher than that of the other regions but converges considerably by 1948.
Figure 1 Infectious mortality, standardised and unstandardised, in US cities by region
a) Infectious mortality rate per 100,000
b) Logged, age-standardised infectious mortality
c) Logged, age-standardised infectious mortality, balanced panel
Notes: In 1a, we report regional medians of mortality rates per 100,000. In 1b and 1c, infectious mortality is standardised to eliminate variation in the age distributions of cities; the trend lines depict the logged ratio of actual to expected infectious mortality, based on the city’s age distribution.
Infectious mortality also declined later in southern cities than in cities in the other regions. This can be seen in Figure 2, where each dot represents a city-year observation and darker dots depict the median city in each year. We divide the data into five periods, corresponding to changes in how diseases were classified by the International List of Causes of Death. We draw linear trends within each period, excluding the flu years of 1918-1920. The figure shows that infectious mortality in cities in the three non-southern regions fell gradually and constantly from 1900 to 1930, accelerating from 1930 through the end of the series. The urban South's decline, in contrast, stayed gradual until the late 1930s, when it became much sharper than that of the other regions.
Figure 2 Logged, age-standardised infectious mortality in US cities by region
Notes: Infectious mortality is standardised to eliminate variation in the age distributions of cities. Each dot represents the logged ratio of actual to expected infectious mortality in one city in one year, based on the city’s age distribution. We plot the median city in each year using darker dots. We draw linear trends within five periods. Each period is defined as a set of years during which the cause of death categories are relatively stable. We omit 1918–1920 to avoid distortions from the flu pandemic.
Our most striking result, however, is that the South's distinctiveness had less to do with causes affecting all residents of southern cities than with the fact that southern cities were populated by greater proportions of black residents, who suffered extreme risks of death from infectious disease in cities in all regions. African Americans in cities across the US often lived in segregated and crowded housing (Du Bois 1908, Galishoff 1985, Acevedo-Garcia 2000, Collins and Thomasson 2004, Roberts 2009, Grigoryeva and Ruef 2015, Eriksson and Niemesh 2016, Zelner et al. 2017), had high poverty rates (Ewbank 1987), and were prevented from accessing many urban social programs (Preston and Haines 1991) and medical innovations (Jayachandran et al. 2010), all of which increased their susceptibility to death from infectious disease (Sen 1998). The risk of death from infectious disease among urban African Americans was so high that it was primarily responsible for regional differences in the infectious mortality of all residents of US cities.
The sample of cities for which we have data on deaths by cause separately for whites and non-whites is smaller than the sample we used to generate Figures 1 and 2. These data span the years 1906 to 1942, excluding 1938. Black urban mortality remained high throughout the period: it took until 1939 for it to fall to the median infectious mortality of urban whites in 1906.
Figure 3a plots logged comparative mortality ratios for all groups in this smaller sample. As in Figure 1, southern mortality exceeds that of the other regions and sharply declines in the late 1930s.
In Figures 3b and 3c, we examine infectious mortality among whites and non-whites separately. The regional differences reported in Figures 1 and 2 are much less pronounced when we examine infectious mortality among whites and non-whites alone. Nationally, the unlogged median comparative mortality ratio of whites from 1906 to 1920 ranged from 1.4 to 3.0 at the height of the flu pandemic. The unlogged median comparative mortality ratio of non-whites, in contrast, never fell below 3.1. African Americans in cities face such a high risk of death from infectious disease that it is as if they lived through the flu pandemic experienced by urban whites in every year from 1906 to 1920.
Figure 3 Logged, age-standardised infectious mortality in US cities by region
a) All groups, cities with racial-group-specific data
b) Whites, cities with racial-group-specific data
c) Nonwhites, cities with racial-group-specific data
Notes: Infectious mortality is standardised to eliminate variation in the age distributions of cities. The trend lines depict the logged ratio of actual to expected infectious mortality, based on the city’s age distribution.
We hope that future research will help to explain the vast inequality in black and white Americans' risk of death from infectious disease in the early 20th century. Roberts (2009) and Zelner et al. (2017) present evidence linking African Americans' high rate of death from tuberculosis in the early 20th century to segregation and crowded housing.
Other well-known causes of black infectious mortality, in contrast, are unlikely to explain the gap in the infectious mortality rates of black and white city dwellers. For instance, by 1900, malaria, a common cause of death in the South (Kitchens 2013), was rare in southern cities (Humphries 2009, Boustan and Margo 2016). Research by Black et al. (2015) and Eriksson and Niemesh (2016) has shown that the Great Migration increased mortality among African Americans at young and old ages. But a large part of this effect was due to the fact that migrants left rural areas for cities, where the risk of death from infectious disease was comparatively high at the beginning of the 20th century (Eriksson and Niemesh 2016). The Great Migration may have accounted for some portion of the regional convergence in urban infectious mortality for all groups, as black migrants, with comparatively high risks of death from infectious disease, left southern cities for cities in other regions. But it should account for a smaller portion of the difference in infectious mortality between urban whites and urban African Americans because black infectious mortality rates in southern cities were similar to black infectious mortality rates in cities in the other regions.
In future work, we plan to study infectious mortality among urban African Americans in closer detail, focusing especially on which causes of death were primarily responsible for their extreme mortality rates.
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