CHICAGO (Reuters) - School shootings in the United States rise as the economy slows and unemployment increases, according to a university study published on Monday.

An aerial view of Sandy Hook elementary School in Newtown, Connecticut, is pictured in this evidence photo released by the Connecticut State Police, December 27, 2013. REUTERS/Connecticut State Police/Handout via Reuters

Over the past 25 years there have been two periods of increased gun violence in U.S. schools and “the timing of these periods significantly correlates with increased economic insecurity,” said researchers from Northwestern University in Evanston, Illinois.

“The link between education and work is central to our expectations about economic opportunity and upward mobility in America,” said John Hagan, a professor of sociology at Northwestern University and one of the authors of the study, which was published in the journal Nature Human Behavior.

“Our study indicates that increases in gun violence in our schools can result from disappointment and despair during periods of increased unemployment, when getting an education does not necessarily lead to finding work.”

Numerous high-profile shooting incidents have highlighted the U.S. debate over gun control measures, but proponents of tighter restrictions have been blocked for years in Washington by the National Rifle Association and other gun rights defenders.

Frequent efforts at gun control have failed despite anger at mass shootings like the killings at an elementary school in Newtown, Connecticut, in 2012.

Combing six datasets to compile a list of 381 school shooting incidents from 1990 to 2013, researchers at Northwestern found two periods of increased gun violence in U.S. schools: 1992 to 1994, and 2007 to 2013. Both periods came when unemployment was rising as the economy faltered.

Shooting incidents from 1992 to 1994 primarily occurred at elementary and high schools, while the 2007-2013 events were largely at postsecondary schools, researchers said.

“We spent days doing nothing but reading about violence at schools, which is quite possibly the saddest thing I’ve had to do for research,” said Adam Pah, a professor at Northwestern’s business school, and the study’s lead author.

To qualify for the study, shootings must have occurred on a school campus, involved students or school employees - either as perpetrators, bystanders or victims - and involved a firearm being discharged, even if by accident.

The average number of deaths across the 381 shootings was one, with just over 6 percent of shootings resulting in three or more deaths. Gun violence at schools, the report said, have not become more deadly over time and most mass shootings occur at locations other than schools.