His work produced evidence that countered and overturned many assumptions, such as that crime rates are higher in urban areas, and increased during post-war periods and economic downturns.
After publishing The Dangerous Class: Crime and Poverty in Columbus, Ohio, 1860-1885 in 1975, Monkkonen moved to the University of California, Los Angeles (UCLA), and continued working there for the rest of his career.
[2][3] His family and colleagues at UCLA established the Eric Monkkonen Fund to support students working in social history.
Using his meticulously constructed database of 1,781 cases in New York City, he found that factors such as poverty, crowding, a corrupt justice system, and riots are not necessarily preconditions to an increase in homicides.
[4] He also found that the murder rate in Los Angeles declined after World War II; it was commonly believed that violence on the battlefield made brought the tendency of violence back at home, but Monkkonen speculated that soldiers returning home were sickened by gunplay and were actually less likely to use weapons to kill, and that they were also more focused on their domestic needs in starting families.