More students reported carrying guns in Chicago than in Los Angeles or New York City over the 2007 to 2013 time period, according to a new study carried out by a team of researchers from the Northwestern University Feinberg School of Medicine. These findings, published in the journal Injury Epidemiology, provide a historical background for the spike in 2016 gun violence in Chicago, which involved mostly youths and young adults. The study results also suggest that while gun carrying increased in Chicago over the 2007 to 2013 time period, it declined rapidly in Los Angeles.

In Chicago, 9 percent of high school freshman and sophomore students reported carrying a gun between 2007 and 2013; in New York and Los Angeles, this percentage was 4 and 6 respectively. The study found that students were more likely to carry a gun when exposed to more violence risk factors, such as doing illegal drugs, being exposed to fights or feeling unsafe in school.

Based on these results, the authors hypothesize that Chicago students, who were carrying guns in 2013 and were in the age group 14-16 were likely involved in Chicago’s gun violence in 2016 and 2017.

In 2016, Chicago’s ‘out of control’ violence produced 3,500 shootings and 762 homicides—the highest since 1996, according to historical data from the Chicago Police Department. These figures represent a 57 percent increase in murders over 2015 and the biggest spike in Chicago in 60 years.

Oft targeted on human rights, China hits back at U.S. gun culture, racism in wake of massacre http://t.co/dEXiQ2wou9 pic.twitter.com/xw1Bp4wbEp — Chicago Tribune (@chicagotribune) June 19, 2015

“Our findings suggest that there is a clear link between the increase in Chicago students carrying guns in 2013 and the city’s spike in gun violence in 2016,” said senior author Joseph Feinglass, professor of general internal medicine, geriatrics and preventive medicine at Feinberg.

“The city was fertile ground for this increase in shootings.”

Researchers arrived at these results based on self-reported data from the national Youth Risk Behavior Surveillance System (YRBS). More than 50,000 respondents participated in this voluntary survey of public high-school students (freshmen and sophomores) from three cities between 2007 and 2013. The study used four biennial waves of the YRBS to investigate gun carrying reported by the students.

“It’s not hard to imagine why more students in Chicago carry guns than the other two cities with significant violence and homicide burden,” said co-author Dr. Karen Sheehan, professor of pediatrics and preventive medicine at Northwestern University Feinberg School of Medicine and a physician at Ann & Robert H. Lurie Children’s Hospital of Chicago.

“Kids in Chicago are experiencing multiple layers of violence and fear of violence in school on a daily basis.”

In this study, researchers created a violence index, which accounted for mental health risk factors (feeling sadness or hopeless) and behavioral health factors (bullying and physical fights at school). This violence index allowed researchers to categorize the most high-risk students and to describe the magnitude of students’ increased likelihood to carry a gun. Chicago students were found to have a significantly higher occurrence of most mental health and behavioral health risk factors compared to students in LA or New York.

“Our findings highlight the ongoing need to address Chicago’s concentrated poverty and unemployment problems, its extreme levels of racial and ethnic segregation and the hopelessness and isolation so many young people feel,” Feinglass said.

In all three cities, more boys (8.4 percent) reported carrying guns than girls (2.5 percent). More African-Americans reported carrying a gun in the previous 30 days (6 percent) compared to Hispanics (5.5 percent) and non-Hispanic whites (3.5 percent).