From the choice of schools to safety to access to green spaces and healthy food, the neighborhood where a child is raised can play a determining role in their future health. And because structural racism can systematically silo nonwhite people in certain neighborhoods, those local factors shape the health of millions of people of color in the United States. Now, census data link Black children’s neighborhoods and mortality rates in the early 20th century, exposing segregation’s devastating impact on health more than 100 years ago.

The study shows segregation drove racial health disparities “not just today, but [also] in the past,” says New York University community psychologist Adolfo Cuevas, who was not involved in the work.

John Parman, an economist at the College of William & Mary, says the new results are striking because they document the impacts even before the makings of the Jim Crow era in the late 19th century, which legalized and enforced racial segregation and is known to have exacerbated health inequities.

A growing body of evidence has shown that, today, neighborhoods with majority nonwhite residents tend to have poorer health—the result of many accumulated social and environmental inequalities such as systematic overcrowding, higher noise levels due to industrial projects, and exposure to toxic hazards. But how early such residential segregation began to affect health was not clear, says J’Mag Karbeah, a health services researcher at the University of Minnesota (UM) who led the new study. “What is really missing is this crucial period post-Emancipation and before the formalization of Jim Crow legislation,” she says.

So Karbeah and J. David Hacker, a demographic historian at UM, set out to correlate early segregation with child mortality, a proxy for the health of the entire population. “If you don’t have a healthy young population, you won’t have healthy working-age adults, [and] you will not have healthy seniors,” Karbeah says. “It’s really predictive of the quality of your society in the next 40 or 50 years.”

The researchers used census data from 1900 and 1910 that were recently processed by the Minnesota Population Center at UM. The lists, which together cover about 168 million people, include information on literacy, race, and whether the individual lived in a rural or urban area. Census takers also asked each surveyed woman who had ever been married how many children she had given birth to and how many were still alive.

From the data, Karbeah and Hacker reconstructed the number of children born in the 5 years before each census to arrive at a sample of nearly 4.7 million Black and white children. Focusing on the South because 90% of the Black population resided there at the time, they compared the mortality rates for Black and white children. They also calculated the spatial distribution of houses headed by Black or white people as a measure of segregation.

The largest mortality gap was in Savannah, Georgia, in 1910, where Black children were 3.2 times more likely to die than their white counterparts, with almost half dying before age 5. To tease out the influence of segregation, the team controlled for socioeconomic and other variables such as literacy, occupation, and unemployment. They found that in 1910, neighborhood segregation as much as doubled the mortality gap between Black and white children in cities, the team reported last week in Population, Space and Place.

Although the researchers couldn’t explain exactly how segregation affects child mortality, children are extremely vulnerable to environmental pollutants as well as to poor sanitation, Karbeah says, all of which tend to go hand in hand with housing segregation. “These are the populations that have been most impacted by these inequities within the neighborhood environment,” she says.