They analyzed data from 126 countries that account for 90% of the world's population and found that people aged 15 to 29 were responsible for a large percentage of homicides.
However, that age group is shrinking and now accounts for just 21% of the global population. The decline in the size of that age group is strongly associated with falling homicide rates, according to the study, published Oct. 9 in the journal PLoS One.
The largest declines in homicides were in regions with lower crime rates.
North America and Western Europe had a 40% decrease in homicides between 1990 and 2015, Asia had a 37.5% decline, while Eastern Europe and Oceania had a 20% decline.
On the other hand, the Latin American homicide rate rose 10% between 1990 and 2015.
In the United States, the homicide rate fell 43% between 1991 and 2000, going from 10 of every 100,000 people dying by homicide to just under six per 100,000.
Previous research has suggested that falling homicide rates are due to such factors as higher rates of imprisonment, legalization of abortion and economic growth, but this study challenges that.
"If crime rates were influenced predominantly by domestic policies and local socioeconomic conditions, there should be greater divergence rather than similarity in crime trends across countries," said study author Mateus Renno Santos, an assistant professor of criminology at the University of South Florida.
"Rather a shared trend suggests that the homicide decline was not predominantly caused by domestic events, but by phenomena that were shared across countries," Renno Santos added in a university news release.
Some attribute Latin America's increased homicide rate to more people retiring later in life, resulting in fewer jobs for the 15-29 age group and a weakened economy. That, in turn, has contributed to a growing drug epidemic, organized crime and social and political instability, the researchers noted.
Learning more about how population age affects crime rates may help policy makers and law enforcement officials better predict and prevent future criminal activity, according to the study authors.