Gotham is a crime-ridden city and Batman has been trying for decades to solve the problem, but the only character who might truly know the solution is none other than Harley Quinn. Harley Quinn has spent a lot of time in Gotham City and just as much time around its criminals, and she finally cracked the code.
Harleen Quinzel is a trained psychologist and was perfect at her job until she fell into the hands of the Joker. After she was manipulated into becoming Harley Quinn, his criminal sidekick, she went on to terrorize Gotham and be a thorn in Batman's side for a very long time. Harley Quinn remained a loyal sidekick to Joker for a very long time, despite Harley's original violent meeting with him. But underneath all the whacky crazy criminal activities she got up to, she did actually solve one of Gotham City's biggest mysteries, why is there so much crime in Gotham City?
In Stjepan Sejic's Harleen #2 Harley goes into detail about her hypothesis on what she believes causes the increased crime rates in Gotham City. She explains that much like soldiers in a warzone, once they've spent so much time in the fight or flight response that their brain begins shutting down their ability to feel empathy in order to better allow them to survive. She continues to explain that because Gotham Citizens have been subjected to such terrorist attacks from the likes of Bane or Joker for so long, their ability to feel empathy may be impacted. Which would make it significantly easier to commit crimes against one another in the name of survival.
Gotham City has suffered many attacks over the years. From the City of Bane to the Joker War, from War Games to The Night of the Owls. It's no wonder that the citizens of Gotham might find themselves in a constant state of fight or flight, to the point where they stop being able to feel for each other. Harley Quinn was the only character so far to be smart enough to actually diagnose this issue, whether Batman will ever be able to find a solution is another matter, but at least there's a place to start now. Even though other cities like Metropolis might be more crime-ridden than Gotham, there's still no denying that Gotham has a very serious problem with violent crime that Batman's current methods, while definitely having positive effects, aren't slowing the tide fast enough.
Batman has been fighting crime in Gotham City for almost 100 real years at this point, while his methods have absolutely been reducing the threat of violent crime, this reduction has been slow. Maybe next time Batman starts to put together a plan to fight crime he could call in Harley Quinn for her expert psychiatric opinion on just where the crime is coming from.