The broad story is familiar: The United States has taken tremendous strides narrowing the racial gaps in life expectancy, disease, and infant mortality.

And yet, it remains murky just how we got here. In a recently prepared chapter for the Oxford Handbook of Economics and Human Biology, economists Leah Boustan and Robert Margo investigated the past century of health disparities between white and black Americans.

“Partly what struck me was how large the changes over time have been,” said Margo, a professor of economics at Boston University. “And yet on many fundamental health outcomes, we’re still in the dark on how big they were, and when they came down and why they came down.”

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Improvements in public health played a large role, as did progress on civil rights and decreases in income inequality — but we don’t fully understand the contributions of each. Nor do we understand, for that matter, why stubborn differences still exist between the health of black and white Americans.

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Take life expectancy. A white baby born in 1900 might live into his or her late 40s; a black baby’s life expectancy was 10 or 15 years shorter, depending on whose data you believe (Boustan and Margo make a case for the optimistic figure).

Early on in the century, life expectancy rocketed upward for both white and black people. The economists credit disease eradication in the South, and sanitation improvements in the North for this change.

“In the early 20th century, the American South was basically a third world country,” Margo said. There was malaria and hookworm everywhere. The northern cities had their own problems involving filth, pollution, and human excrement.

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The subsequent revolutions in public health and sanitation lifted all boats, but also widened the black-white gap. The economists write that white Americans saw the greatest gains in life expectancy in the past century, which occurred between 1900 and 1940. For black Americans, the changes would happen later, in two distinct periods.

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First was the post-war era between 1940 and 1950; this was a time when black people were staying longer in school, and starting to earn more on the job market. Black schools had also been increasing in quality. Health and education are of course entwined — you have to be healthy to work and go to school. But research suggests that for people around this time, an additional year of schooling led to at least a 17 percent decrease in the chance that someone would die in the subsequent decade.

Black Americans also caught up significantly after the civil rights reforms of the 1960s. The Civil Rights Act of 1964 forced hospitals to integrate if they wanted access to federal money. Much of the increase in life expectancy can been seen in how black infant mortality rates plummeted around this time, as more black mothers began to deliver their babies in hospitals. A trio of economists have argued that between 1965 and 1975 alone, desegregation saved the lives of 5,000-7,000 black babies.

These days, the differences between black Americans and white Americans tend to be smaller in absolute terms, but can loom larger in relative terms. For instance, in 1910, black men aged 60-74 were about 70 percent more likely to suffer from arteriosclerosis, the hardening of the arteries that accompanies heart disease and high blood pressure. These days, they’re more than two and half times as likely. The gap in infant mortality has closed, but black babies still die at a rate 140 percent higher than white babies.

Boustan and Margo’s exploration concludes on this mystery. The big disparities of the past, they came down for pretty understandable reasons: “Improvements in public health, rising income, things like this,” he said. “Then we get these residual differences that we live with today, and they’re very hard to account for.“ The differences, granted, are getting smaller, but are irritatingly persistent. This resembles the broader picture of racial progress in America. It’s hard to imagine how far the nation has come, the leaps that have been made. The last step, always, is the hardest.

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