Paris (AFP) - Surges in US school and university shootings strongly overlap with periods of economic insecurity, said researchers Monday, offering a partial explanation for a phenomenon experts have been grappling to understand.
Researchers from Illinois gathered statistics on shootings at schools, universities and colleges from 1990 to 2013, and compared these to unemployment and other economic data for the same period.
There were more shootings in periods of economic hardship, they said.
"When it becomes more difficult for people coming out of school to find jobs, the rate of gun violence at schools increases," said a statement from Northwestern University, whose experts participated in the study.
Sociologist John Hagan, who co-authored the paper, said gun violence appears to result from "disappointment and despair" in periods "when getting an education does not necessarily lead to finding work."
The team compiled a dataset of 381 shootings at primary and secondary schools over 23 years, as well as universities and colleges.
To be included, the incident had to involve a firearm being discharged, even by accident, had to occur on a school campus, and had to involve students or school employees.
There was one death per shooting on average, and two or more deaths in 6.3 percent of cases, the study found. Gang-related violence accounted for 6.6 percent of shootings.
The team compared the shooting data to statistics on joblessness, consumer confidence and rates of foreclosure -- taking possession of a mortgaged property for which loan repayments have fallen behind.
"In the last 25 years, there have been two periods of elevated gun violence at schools in the United States and the timing of these periods significantly correlates with increased economic insecurity," the team wrote.
These were the periods 1992-94 and 2007-13.
The results suggest it may be useful to boost vigilance at colleges and universities during periods of heightened unemployment, the researchers said.
But while the study may point to a cause for school shooting surges, "it does not explain why such a... rate of gun violence should exist at all in the United States", they added.
Other key findings included that school shootings have not become deadlier over time, and that most were targeted at a specific person.
The study was published in the journal Nature Human Behaviour.