From FBI Uniform Crime Reports 2000:

"Historically, the causes and origins of crime have been the subjects of investigation by varied disciplines. Some factors which are known to affect the volume and type of crime occuring from place to place are:
* Population density and degree of urbanization.
* Variations in composition of the population, particularly youth concentration.
* Stability of population with respect to residents' mobility, commuting patterns and transient factors.
* Modes of transportation and highway system.
* Economic conditions, including median income, poverty level, and job availability.
* Cultural factors and educational, recreational, and religious characteristics.
* Family conditions with respect to divorce and family cohesiveness.
* Climate.
* Effective strength of law enforcement agencies.
* Administrative and investigative emphases of law enforcement.
* Policies of other components of the criminal justice system (i.e., prosecutorial, judicial, correctional, and probational)
* Citizens' attitudes toward crime.
* Crime reporting practices of the citizenry."

Source:
[url=http://www.fbi.gov/ucr/00cius.htm]Uniform Crime Reports - Crime In The United States 2000, Forward Section[/url]

Quoted:

What is conspicuous by its absence from this list of factors that affect volume and type of crime occuring?

Quoted:

Armed citizens.

Quoted:
-snip-

"Owning a handgun doesn't make you armed any more than owning a guitar makes you a musician."  Col. Jeff Cooper