"The link between education and work is central to our expectations about economic opportunity and upward mobility in America," said Hagan, the John D. and Catherine T. MacArthur Professor of Sociology in the Weinberg College of Arts and Sciences. "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."

Frequent school shootings have been a major concern in American society for decades, but the causes have defied understanding. The Northwestern researchers used data from 1990 to 2013 on both gun violence in U.S. schools and economic metrics, including unemployment, to get some answers.

"Our findings highlight the importance of economic opportunity for the next generation and suggest there are proactive actions we could take as a society to help decrease the frequency of gun violence," said Pah, clinical assistant professor of management and organizations at the Kellogg School of Management.

Other key findings include:

• While Chicago is singled out in the study as one of the six cities with the most incidents from 1990 to 2013, Chicago schools are not any more dangerous than schools in other large cities.
• Gun violence at schools has not become more deadly over time.
• Most shootings are targeted, with the shooter intending to harm a specific person.
• Gang-related violence and lone mass shooters comprise only small fractions of the gun violence that occurs at U.S. schools. Gang-related violence constitutes 6.6% of all incidents.
• The results suggest that during periods of heightened unemployment, increased gun violence may be a growing risk in American college and university settings.

The study, "Economic Insecurity and the Rise in Gun Violence at US Schools," was published Jan. 30 by the journal Nature Human Behaviour.

"Our work helps us understand why the frequency of gun violence at schools changes, not necessarily why gun violence at schools in the United States exists at all," said Amaral, professor of chemical and biological engineering in the McCormick School of Engineering.

In the last 25 years, there have been two elevated periods of gun violence at U.S. schools, the researchers found; 2007-2013 was largely due to events at postsecondary schools while 1992-1994 more often involved events at K-12 schools.