Robbery is a pretty good "index crime." For instance, in 1900, there was about 1 robbery per day in all of England (source: http://www.parliament.uk/documents/commons/lib/research/rp99...). This is roughly a factor of 35 lower than current reported crime rates (same source) in the ol' Sceptered Island - assuming you trust HMG's statistics. And all crime statistics in all countries everywhere are generally admitted by all informed observers to be utterly buggered to hell. (example: http://www.villagevoice.com/2010-05-04/news/the-nypd-tapes-i...)
But what is it an index of? What hidden variable influences robbery numbers? Our understanding of crime is incomplete without considering the reasons why crimes are committed (without resorting to the fundamental attribution error by branding those who commit crimes as "criminals" by nature). Some questions to ask about the given example of robbery:
Why do the people committing robberies feel the need to commit them?
What makes robbery more rewarding than other potential sources of money or thrills?
Have robberies increased because the average person now has a lot more of value to take?
How many robberies are indirectly caused by the criminalization of lesser offenses? (e.g. criminalization of drug M -> price of drug goes up + M user loses job -> M user has to resort to robbery to survive)
I don't care to be able to say that England has a worse crime rate than the U.S. I think given the sociological makeup, [lack of] urban planning, poorly run social services, and an overarching cultural dislike of some forms of authority, it would be surprising if any developed country had crime rates as bad as we do.
While not ultimate causes, there are some not so hidden variables:
Population density. There's some idea that it increases stress, which increases propensity to commit crimes.
Sale of contraband, particularly drugs, fuels competition for territory and customers which, in an illegal market, is primarily dealt with through violence.
Brainstorming about gang violence, it seems to me to be mostly about a culture that demands unrealistic levels of respect, and demands taking advantage of any perceived weakness. If someone disrespects a gang member, it's my understanding that they have to retaliate with violence. If they don't, or if they're seen as weak for some other reason, that's an invitation for an opposing gang to put the weak out of their misery. You can't have a stable crime-free area with factions like that running around.
While I wasn't referring specifically to gang violence in my comment about robberies, I have to say your final paragraph about showing weakness sounds an awful lot like international politics.
When I was writing that I thought of North Korea and Iran. Maybe any entity (human, gang, corporation, or leviathan [the state]), when it doesn't feel like it's an accepted part of a larger group, degenerates into displaying that sort of behavior?
But what is it an index of? What hidden variable influences robbery numbers? Our understanding of crime is incomplete without considering the reasons why crimes are committed (without resorting to the fundamental attribution error by branding those who commit crimes as "criminals" by nature). Some questions to ask about the given example of robbery:
Why do the people committing robberies feel the need to commit them?
What makes robbery more rewarding than other potential sources of money or thrills?
Have robberies increased because the average person now has a lot more of value to take?
How many robberies are indirectly caused by the criminalization of lesser offenses? (e.g. criminalization of drug M -> price of drug goes up + M user loses job -> M user has to resort to robbery to survive)