Some facts from the Harvard School of Public Health about the correlation between gun availability and homicides: Homicide - Firearms Research - Harvard Injury Control Research Center - Harvard School of Public Health.
1. Where there are more guns there is more homicide (literature review).
Our review of the academic literature found that a broad array of evidence indicates that gun availability is a risk factor for homicide, both in the United States and across high-income countries. Case-control studies, ecological time-series and cross-sectional studies indicate that in homes, cities, states and regions in the US, where there are more guns, both men and women are at higher risk for homicide, particularly firearm homicide.
Hepburn, Lisa; Hemenway, David. Firearm availability and homicide: A review of the literature. Aggression and Violent Behavior: A Review Journal. 2004; 9:417-40.
2. Across high-income nations, more guns = more homicide.
We analyzed the relationship between homicide and gun availability using data from 26 developed countries from the early 1990s. We found that across developed countries, where guns are more available, there are more homicides. These results often hold even when the United States is excluded.
Hemenway, David; Miller, Matthew. Firearm availability and homicide rates across 26 high income countries. Journal of Trauma. 2000; 49:985-88.
3. Across states, more guns = more homicide
Using a validated proxy for firearm ownership, we analyzed the relationship between firearm availability and homicide across 50 states over a ten year period (1988-1997).
After controlling for poverty and urbanization, for every age group, people in states with many guns have elevated rates of homicide, particularly firearm homicide.
Miller, Matthew; Azrael, Deborah; Hemenway, David. Household firearm ownership levels and homicide rates across U.S. regions and states, 1988-1997. American Journal of Public Health. 2002: 92:1988-1993.
4. Across states, more guns = more homicide (2)
Using survey data on rates of household gun ownership, we examined the association between gun availability and homicide across states, 2001-2003. We found that states with higher levels of household gun ownership had higher rates of firearm homicide and overall homicide. This relationship held for both genders and all age groups, after accounting for rates of aggravated assault, robbery, unemployment, urbanization, alcohol consumption, and resource deprivation (e.g., poverty). There was no association between gun prevalence and non-firearm homicide.
Miller, Matthew; Azrael, Deborah; Hemenway, David. State-level homicide victimization rates in the U.S. in relation to survey measures of household firearm ownership, 2001-2003. Social Science and Medicine. 2007; 64:656-64.