The Great Depression was a difficult, life-altering period in the United States when millions of people struggled to find work and get by. Despite the tough times, the average life spans of Americans actually increased.

In fact, historical research shows that during the 20th century, increases in U.S. mortality often occurred during times of economic prosperity, while decreases occurred during economic depressions or recessions.

In the first few years after the 1929 stock market crash, the only major cause of death that increased was suicide, says José A. Tapia Granados, a professor of politics at Drexel University and co-author of a 2009 research paper in PNAS about life and death during the Great Depression. While suicides went up, Tapia found that deaths from cardiovascular and renal diseases stabilized between 1930 and 1932, the worst years of the depression. Traffic deaths dropped in 1932. Deaths from tuberculosis, the flu and pneumonia also declined.

As a result, the average U.S. life expectancy rose from about 57 in 1929 to 63 in 1933. In both decades, people of color had a lower life average expectancy than white people. Yet when the depression hit, the average life expectancy for people of color rose more quickly than that of whites, increasing by about eight years from 1929 to 1933.

There are no firm answers as to why Americans lived longer during the worst years of the depression, but scholars have made some suggestions. Take traffic deaths: Car use increased during the 1920s, and with it, so too did traffic-related deaths. One possible explanation for their decline in the 1930s is that, with higher rates of unemployment, there were just fewer people on the road. Fewer people could afford to own cars, too—as demonstrated by a famous picture (above) of a man trying to sell his car after losing his money on the stock market.

There’s also research suggesting that during U.S. economic expansions, people smoke more, experience more stress and get less sleep. All these factors can have a negative impact on health. This could apply not just to the Great Depression, but other economic downturns in the 20th century. In 2018, Tapia co-authored another paper in the American Journal of Epidemiology that looked at data from 1985 to 2011, a period that covered three recessions.

“What we found in this paper is that a number of things that are usually thought about unemployed people—well, apparently they are not true,” he says. Although the unemployed people in the study had higher levels of depression, they had lower blood pressure on average. They also did not smoke or drink more than employed people. In fact, Tapia notes that cigarette sales have historically risen when the economy is doing well and declined when it is not.

Longer life expectancy during periods of economic decline was noted as early as the 1920s when William Ogburn and Dorothy Thomas made this observation using American and British data. In 1977, Joseph Eyer revived this theory with a sensationally-titled paper, “Prosperity as a Cause of Death.” Today, scholars have seen similar trends in Europe, and there is some debate as to what this means.

One argument is that a rise and fall of mortality with the economy reflects a “lag” in effects on people’s health. In this scenario, death rates would be higher in a good economy because of the poor health conditions people experienced during a previous recession. And in turn, mortality would be lower in a bad economy because of the good conditions people experienced during the previous economic expansion.

Research has also pointed out that a “good” economy doesn’t mean living conditions are “good” for everyone. Increased economic productivity often creates more pollution, which harms those who have the least access to health services and safe housing. Infants in particular are susceptible to poor environmental conditions; so higher levels of factors like air pollution can increase infant mortality.

There are no simple answers as to why the average lifespan increased during the Great Depression, or why U.S. mortality has continued to rise and fall with the economy. But it does counter assumptions that as the economy goes, so goes the health of a nation.