Moron density and violence: I haven't been particularly impressed with IQ's power to explain criminality. The effect always seems to be very modest.
One problem is that criminologists never examine macrolevel relationships. I cannot recall a single published study that assesses the relationship across geographical units. (I'm sure they are out there somewhere--I just haven't seen one).
I used Audacious' estimates of state-level IQs and correlated this with homicide rates averaged over the years 1999-2005. (Homicide is a rare event--criminologists routinely aggregate over several years).
The size of the Pearson correlation? It's -.80!!! That means that 64% of state-level homicide rates is explained by average differences in IQ. Even percent black explains only half the variance.
For example, New Hampshire has a mean IQ of 101, and 0.6 homicides per 100k population per year. By contrast, Louisiana's IQ is 95, and the homicide rate is 9.7.
So why are these results so much stronger than those found in individual-level studies? Well, macrolevel studies typically reduce measurement error, thus strengthening the correlation. In addition, studies typically focus on minor offenses, like delinquency, and it might be that IQ plays a more important role in serious crime.
According to research, most violence is mutual combat, so it won't usually happen unless two idiots cross paths. The higher the density of morons, the more frequent the occasions where conflict escalates into violence. Plus, a community of dolts might generate of culture of aggressiveness which would exacerbate the problem.
Unfortunately, mass immigration from low IQ countries is increasing moron density, so in the long run we can expect a more criminal society.