UNIVERSITY PARK, Pa. — Poor mental health ranks as one of the costliest forms of sickness for U.S. workers and may sap billions of dollars from the country's income growth, according to a team of researchers.

In an analysis of economic and demographic data from 2008 to 2014, the researchers found that a single extra poor mental health day in a month was associated with a 1.84 percent drop in the per capita real income growth rate, resulting in $53 billion less total income each year, said Stephan Goetz, professor of agricultural and regional economics, Penn State, and director of the Northeast Regional Center for Rural Development.

"This starts to give us an idea of what the gain could be, if we did spend more money to help people with poor mental health," said Goetz, who worked with Meri Davlasheridze, assistant professor and economist, Texas A&M University at Galveston and Yicheol Han, postdoctoral scholar in agricultural economics, sociology and education, Penn State.

Poor mental health days refer to days when people describe their mental health as not good and could include conditions such as depression, anxiety, stress and problems with emotions, according to the researchers, who report their findings in a current issue of the Review of Regional Studies. The measure does not include diagnosed mental illnesses.

To give some sense of the size of the problem, the researchers added that the global economic cost of mental illness is expected to be more than $16 trillion over the next 20 years, which is more than the cost of any other non-communicable disease.

The effect is stronger in rural counties, which tend to be poorer than urban counties. A poor mental health day in rural counties was associated with a reduction of 2.3 percent in income growth, compared with only a .87 percent reduction in urban counties.

"That's an interesting finding in itself, too, because poorer counties already have so many factors going against them," said Goetz. "If poor mental health days have a bigger impact in these poorer counties, it suggests that they would have an even harder time keeping up with the wealthy counties."

Urban counties might have more resources for people struggling with poor mental health and conditions, according to Goetz. These communities typically have more mental treatment facilities, as well.

"We think this difference between urban and rural counties might exist because of the better services that are available for the mentally distressed in the urban counties, which are typically the wealthier counties," he said. "In an urban county, you might have a mental health center, you may have more resources to tap into to help get you through the bad days, and there may be more mental health professionals. In a rural area, you're less likely to have access to those types of resources."

The researchers suggest that investing in mental health resources may be one way of lowering the economic costs of poor mental health, particularly in the harder-hit rural counties.

Goetz cautioned that the period covered in the study was a particularly tumultuous time for the U.S. economy and could have an effect on the findings. To further test the effect of poor mental health days on income growth, he suggested researchers study the relationship over longer time periods and in different economic conditions.

The researchers used county-level data from a number of public sources, including the Bureau of Economic Analysis, the U.S. Census Bureau and the Northeast Regional Center for Rural Development. The mental health day data was drawn from County Health Rankings.