Using data from Richard Lynn and the recent WHO study, the correlation between national level IQ and the homicide rate across 181 countries is -.51. This is a strong association, so smarter countries are much less violent. The only cross-national correlate of homicide that is this strong is income inequality (more unequal countries are more violent). In my data set, the correlation between the GINI inequality index and homicide is .48.
UPDATE: Since I mentioned inequality, any thoughts on how to explain the correlation? The inequality-homicide link is reliable in macrolevel research. Sociologists claim that a large gap between the wealthy and the poor generates anger and resentment which low-status people take out on convenient, low-risk targets. It doesn't seem plausible to me that Bill Gates is an important cause of black-on-black ghetto violence. Compared to Europe, the US has a fairly high level of inequality, but racial diversity probably explains its inequality-homicide correlation. As for the cross-national link, is it that poorly endowed populations are both violent and unlikely to create more equal arrangements? I'll have to see how much the inequality-homicide link is reduced when adjusted for mean IQ.
Back to the US question: if American liberals are so concerned about large gaps between the wealthy and the poor, why would they invite large numbers of both Chinese and Mexican immigrants? At least the immigrants of a century ago were similar (with the exception of Jews).