In 1974 the murder rate in the United States was about 40 times that of Britain. Over time, the two nations have consistently differed by a factor of more than 10. Common hypotheses focus on the widespread availability of handguns in the United States, on its diverse ethnic and racial mix, on the cultural values of the frontier, and to feelings of deprivation among poor people. However, each hypothesis is insufficient. However, analysis of data from the 19th century shows that the two nations were far closer in homicide rates than they have been ever since. After the Civil War the homicide rate in New York City declined, but that of Britain declined much more sharply. Part of the difference resulted from the increase of the use of guns in homicides as a result of the gun culture that arose from the Civil War. In addition, punishment in New York City was feeble. Punishment appeared to depend on whether or not the killing appeared to be reasonable in terms of social propriety. Consistent and vigorous punishment as practiced in other nations might have reduced homicide rates further. Tables, figure, and reference notes.