ONE has always heard that the United States is more violent than other nations, but the statistics still come as a shock. To document American exceptionalism, David T.  Courtwright includes comparative homicide figures for a group of 16 developed nations from the mid 1980s. The homicide rates per 100,000 for males aged 15 through 24 show a gradual increase from .3 (among the refined Austrians) to 5.0 (among the perfervid Scots), with the United States soaring off the chart at 21.9.