Criminologists, sociologists, and public-health scholars have devoted enormous attention to the role of firearms in violence, particularly regarding American homicide rates, but historians have been less inclined to examine the impact of firearms, especially their availability, on changing patterns of violence. Instead, legal and criminal-justice historians have emphasized the ways in which institutional, cultural, political, and social changes have fueled shifts in levels of murder. An analysis of the rich homicide case files and newspaper accounts of gun violence in early twentieth-century New Orleans, however, confirms the theory of “weapon instrumentality”—that homicide rates tend to soar whenever and wherever firearms abound and to decrease when guns are in shorter supply.

White killers of African Americans in New Orleans between 1921 and 1945, nearly half of whom were policemen, insisted that they acted in self-defense, only after their victims had threatened them, often by reaching for weapons. But many of their victims were unarmed. The conventional interpretation is that white residents invoked a formulaic justification of self-defense to mask their real intention, to uphold the city's racial hierarchy. Recent studies by cognition researchers, however, suggest a more complicated interpretation—that endemic racism can influence how the brain processes information, even to the extent of causing people to see a weapon where none exists.

The American police have a long history of relying on violence. In Chicago, however, the widespread use of deadly force did not date to the rough-hewn, early days of nineteenth-century policing. Rather, an analysis of more than 300 killings by Chicago policemen from 1875 through 1920 reveals that the frequent resort to deadly force began during the Progressive era. In this period, a formal shoot-to-kill, law-enforcement strategy developed in response both to a local crime wave and, ironically, to reformers' demands for more vigorous and more professional crime fighting.

Scholars often treat family violence as a single category and argue that domestic violence typically reflects conflict over gender roles. Such a focus has been well placed. But if data on domestic homicide in Chicago from 1875 to 1920 are disaggregated by ethnicity and race, important patterns emerge. Domestic homicide, for example, assumed culturally specific forms. German immigrants, Italian immigrants, and African-American Chicagoans killed loved ones for different reasons, at different rates, and with different family members involved. Although the violence revolved around challenges to gender identity and expectations, each group defined such challenges in distinct ways, reflecting a complex blend of cultural assumptions and material circumstances.