U.S. researchers say they were surprised to find life expectancy increased by 6.2 years during the Great Depression.

The study, published in the Proceedings of the National Academy of Sciences, finds life expectancy rose from age 57.1 in 1929 to 63.3 years in 1932. The increase occurred for both men and women and whites and non-whites.

"The finding is strong and counter-intuitive," study lead author Tapia Granados of the University of Michigan Institute for Social Research in Ann Arbor says in a statement."Most people assume that periods of high unemployment are harmful to health."

The study was based on historical life expectancy and mortality data for 1920-1940. The researchers found that while population health generally improved during the four years of the Great Depression and during recessions in 1921 and 1938, mortality increased and life expectancy declined during periods of strong economic expansion, such as 1923, 1926, 1929 and 1936-1937.

The association between improving health and economic slowdowns was true for all ages, and for every major cause of death except one -- suicide, the study says.