Well then CR, it seems you'd be surprised to learn that "Research has consistently found lower cognitive ability to be related to increased risk for violent and other antisocial behaviour", as noted in the abstract of
this study and substantiated by the previous research cited in its introduction.
This study, as with most similar studies, looks at
criminals and tries to make the erroneous connection between IQ and crime. That is tantamount to conducting a survey of people exiting from a football game and using the results to make some astonishing claim like, "Ninety-five percent of all people surveyed like football." The particular study you cited looks very narrowly at Swedish brothers and half-brothers in an effort, it seems, to discuss the "nature vs. nurture" issue as well. This "sampling" is not random at all, and is, therefore, flawed.
The basic flaw in this
type of study is that persons with a lower IQ are much more likely to be apprehended for their crimes. So, the sampling isn't a true random sampling. In fact, using criminals as the base is skewed from the start. Also, in the study cited, the statistics relating to the population as a whole are sketchy, at best. The subjects were chosen from the "conscription" records, and many people were excluded. A truly random sample, necessary for a valid study, might be impossible for the reason I mentioned above - the more intelligent criminal is less likely to be apprehended.
My observations are based on a random sampling of students - those I taught. Based on my sample, my observations were quite different from the studies of criminals described in your link - and in numerous similar studies, all of which contribute to the false (IMO) assumption that a low IQ means that the individual is more likely to become a criminal. That was not my experience at all. Students who became violent were not any more likely to be of lower IQ than of normal IQ or even higher IQ. It appeared to me that violent students were pretty much distributed in a Bell curve across the intelligence spectrum and every other spectrum!
I am firmly convinced, based on my 25 years' teaching experience, that, were a
truly random sampling of people guilty of violent crimes ever studied, the results would show that there is no one thing that causes the violence - except maybe a physio-neurological disorder. And, anyone, of any intelligence level or socioeconomic status, can have such a disorder. If I were to try to pinpoint a "cause" of violence, it would be a physio-neurological condition, not low intelligence or socioeconomic factors.