Kevin Drum has an important point about levels of violence in the Middle East and North Africa (MENA). First, an image showing the time frame in which MENA countries phased out leaded gasoline:
Drum explains why this is important:
[T]here's a lot of evidence that leaded gasoline produced a wave of violent crime between 1960-1990 in the developed world, and that the introduction of unleaded gasoline eliminated that wave and eventually brought crime rates down nearly to 1960 levels. In most developed countries, leaded gasoline was phased out starting around the mid-70s, which benefited children born after that. When those children reached their late teenage years in the early 90s, they were much less prone to impulsiveness and aggression, which led to lower crime rates.
But not every part of the world followed that timetable. In particular, leaded gasoline continued to be used in the Middle East up through the late 90s. Egypt began phasing it out in 1998, and most other countries followed over the next decade or so. Only a few—including Iraq and Afghanistan—still sell significant amounts of leaded gasoline.
Since lead poisoning affects infants, its affects show up about 18-20 years later. What this means is that in the bright red countries, the cohort of kids who reach their late teen years around 2020 should be significantly less aggressive and violent than previous cohorts. Around 2025 the countries in lighter red will join them. Around 2030 the countries in pink will join. By 2040 or so, the process will be complete.
If you want the longer version of Drum's argument, go this this article, which contains ample citations and further sources. Suffice it to say that I am convinced lead exposure is the main environmental factor in increasing violent crime.
As for the picture, you will no doubt notice that these are precisely the countries from which young males streamed into German in 2015. They are, of course, committing large numbers of all kinds of crimes here in Germany, as you would expect from young males anywhere. That is not open to dispute.
It's still too early to determine whether they are committing proportionately more violent crimes than people who grew up in (relatively) lead-free Germany. I have my suspicions that there are a large number of mentally-disturbed people among the new arrivals, judging by thousands of incidents of criminal and/or bizarre behavior, including public masturbation. Childhood lead exposure leads to lifelong permanent increases in impulsive behavior, and what could be more impulsive than deciding to whip out your penis and masturbate in front of a crowd of strangers?
In any case, if the lead-crime hypothesis is right, and I think it is, then young males from these countries will show an above-average tendency to commit impulsive violent actions which will probably persist until their testosterone levels drop when they reach their 40s. Of course, this doesn't mean most of them will commit violent crimes, only a minority will. Lead exposure varies considerably by geography. Nor does lead exposure turn everyone it affects into monsters, of course. It has marginal, population-wide effects of increasing the incidence of violent actions in a given cohort. But still, the increase is very noticeable and very measurable.
It seems like this is the sort of thing policymakers might want to have considered before letting hundreds of thousands of young males from these areas into the country, no?