What better time to steal something than when an entire city is fawning over a sporting event?

F0otball games are associated with upticks in city crime, according to a recent study (pdf), which observed crime rates in eight separate cities—Detroit, Miami, New Orleans, Newark, Philadelphia, St. Louis, Baltimore, and Washington—over the course of a two-year period. Specifically, the study found that days on which cities hosted home games for their respective professional football teams coincided with a nearly 3 percent increase in total crime, including a more than 4 percent increase in larceny and almost 7 percent increase in the number of car thefts.

"NFL home games are correlated with a higher incidence of crime compared to non-game days or days when the team is playing an away game in another city," the researchers wrote.

Early afternoon games, which begin at 1pm eastern time, are the most closely connected to higher crime rates—they were found to be associated with a 4 percent increase in both total crime and economic crime, considerably more than those beginning in the late afternoon or night. The association is likely a reflection of a more general trend, which shows criminals tend to act during the daytime (roughly 40 percent of property crime occurs before 6pm, while only about 13 percent takes place between then and midnight, according to the 2008 National Criminal Victimization Survey).

There are a few reasons why football games present so many opportunities for criminals.

For one, criminals likely find themselves with more options in crowded places where people gather to watch games. Consider car thefts, the form of criminal activity most significantly correlated with NFL games, for instance.

"If stadium, restaurant, bar, and other parking lots are full of cars, it will be easier for thieves to find suitable cars to steal," the researchers wrote.

The same can be said for the sheer concentration of people, and, therefore, personal belongings. A sea of targets might mean an increased likelihood of finding one that is particularly susceptible for theft.

But large gatherings—of both cars and people—don't merely present options; they also tempt criminals with the potential for increased stealth. Large crowds mean criminals more easily lurk, and even loot, unnoticed.

"A large gathering of people on game day increases the number of potential targets and may also reduce the likelihood of criminal apprehension, as criminals can blend more easily into larger crowds," the authors note.

Similar trends have been noted for large gatherings for other sports or public events. Previous research has, for instance, found a correlation between crime near not only the North Carolina's professional football team stadium during home football games, but also its professional basketball team's stadium during home basketball games. But football games tend to make for particularly large gatherings, likely exacerbating the problem.

The study is unique in that it links NFL games to property theft, specifically, but it's hardly the first to find a connection between football games and increases in crime, more generally. A 2008 study concluded that college football games are associated with increases in both assaults and vandalism. A separate study from 2011 found that domestic violence increases during NFL home games, especially following upset losses by the home team.

It's worth noting that while some crimes—specifically larceny and motor vehicle robberies—were found to increase considerably during home football games, others, including burglary and robbery, appeared unchanged.

Still, the cost of the upticks in criminal activity during football games is fairly significant. Using Baltimore as a benchmark, the researcher's estimated the nearly 3 percent increase in total crime amounts to more than $85,000 per game day, or nearly $700,000 per year, per city.