Lead causes crime.
"What!?" You exclaim. "Not lead. Why I've known lead for years. It's been in my car battery for decades and in my gas before that! Lead would never be caught committing a misdemeanor much less a felony! Surely you mean gold, or silver, or one of those other unsavory elements like uranium."
I admit, I was as surprised as you are now. However, the research is compelling. Let's start at the beginning. If we're going to try and make the case that lead causes crime than we need to have a mechanism for that effect. We need evidence to support the hypothesis and it would be nice to have experiments designed to eliminate extraneous variables. So here it is step by step.
How does lead drive crime rates?
The mechanism of action is relatively simple. During early developmental years children exposed to even small amounts of environmental lead exhibit neurodevelopmental damage including decreased IQ, reduced hand eye coordination, decreased reaction time and lowered information processing capacity. This neurological damage early in life is compounded as the child develops and leads to poor decision making abilities later in life. Poor decision making then leads to an increased likelihood of criminal acts during the late teens and early twenties.
It's a simple but lengthy chain of events. If we're to believe it has any merit it's necessary to provide evidence for each link along the way.
Therefore premise 1: children are exposed to lead. Environmental lead exposure is universal to varying degrees. Most commonly in the United States high doses of lead are attributed to lead paint in older buildings. However, lead solder in plumbing can leach into tap water, lead from leaded gasoline persists in soil for years and occasionally foreign made toys have higher than allowed lead content. All of these factors are potential methods of exposure to lead for infants. It is important to note however that like all toxins lead does require an abnormal dose in order to cause harm. In other words although everyone is exposed to environmental lead, for most of us it's nothing to be concerned about. Still, a mechanism for exposure of infants to lead is evident. Beyond that, blood tests have been conducted on infants and have at times found lead levels in the blood that are thought to lead to neurodevelopmental damage.
Premise 2: Exposure to lead causes neurological damage. Numerous studies have been conducted regarding the effects of lead on both human and animal biology. Rather than speak at length regarding whether or not lead causes neurological damage, I am simply going to assure you that it does and provide sources you may investigate further if you have doubts. Sources are here, here, and here. Further studies supporting the premise are easily found online if desired.
Premise 3: Neurological damage of the type lead causes leads to an increased probability of committing crimes later in life. This is likely the weakest section of the chain. Studies have shown that lower IQ is correlated with a higher chance of committing criminal acts. However, we can not support a case for causation by utilizing another correlation which may itself not indicate causation. Hypothetically, we could assume that decreased IQ leads to fewer opportunities for adequate income which may lead to crime committed out of desperation. Ultimately I was unable to locate research sufficient to unequivocally support the idea that lowered IQ leads to an increased likelihood of criminal acts. However, the association seems likely based upon recent research.
It seems research has for the most part supported this chain of events. Lead is found in infants in rough proportion to the lead that is found in their environments. Lead is found in materials that infants commonly come in contact with. Lead is well known to cause neurological damage. Finally, studies have indicated that decreased IQ (which can be a result of the neurological damage lead causes) is correlated with a higher probability of criminal activity. Everything seems plausible, but what real world evidence is there?
Evidence for the lead/crime correlation
Before diving into this section I want to make sure due credit is given. The large majority of the following cited research is directly authored by or based upon work done by Rick Nevin. He is to the best of my knowledge the originator of the idea that lead exposure in children drove the crime increase of the 70s and 80s as well as the subsequent decrease in crime rates in the following decades.
Given the idea that environmental lead causes heightened crime rates what evidence could we look for that might give credence to our hypothesis? It seems that if higher lead levels cause more criminal acts than we simply need to look at different locations and time periods and see if those with higher lead levels also have more crime. Since we're looking at how childhood exposure effects crime later in life we'll of course need to allow time for those children to grow up and become potential criminals. Therefore we need to look at crime rates at a given time versus lead levels sixteen to twenty years ago.
This is exactly the sort of analysis Nevin has conducted on numerous countries, locations and time periods. I encourage you to read his publications here and here which briefly summarize the breadth of his work. However, to compact it even further; he has found that decreased lead exposure due to leaded gasoline bans and leaded paint bans has been highly correlated with decrease crime rates starting in the 90s (again with a sixteen to twenty year time lag). This correlation holds true for different nations as well as for states that implemented leaded gasoline bans earlier than the federal ban. Additionally, areas with higher leaded paint content (mostly inner cities with older buildings) demonstrate higher crime rates.
Further given the United States historic lead levels which peaked between 1965 and 1975 we should find a population born during that time period which is particularly inclined towards crime. In fact that is exactly what evidence seems to indicate. From 1970 to 2003 the property crime rate was relatively static. However, the demographics of the perpetrators shifted dramatically. Youth property crime fell dramatically during the time period (nearly 50%) while crimes committed by those over 24 increased by over 50%. If not for the crimes of those born during years of high lead levels the property crime rate would have decreased dramatically.
Overall the evidence is compelling. However, one study in particular was influential. This study followed individuals from prenatal care into adulthood. Participants were initially mothers from Cincinnati who resided in areas at risk for high environmental lead levels. Their children were periodically tested for blood lead levels. The results indicated that higher levels of blood lead were associated with higher rates of violent crime. The significance of this study lies in the fact that all of the participants were from a similar area and of similar economic status. Mitigating the effects of these extraneous variables lends a great deal to the credibility of Nevin's hypothesis.
No one is attempting to make a case that lead exposure causes all or even most of crimes. Higher end estimates guess that lead exposure is a factor in approximately twenty percent of criminal acts. However, given the extreme costs of catching, trying and housing criminals even a small reduction in crime rates represents a monumental savings. This study in particular proposes that reducing blood lead levels in infants to under 1 microg/dL would result in savings of 1.2 trillion dollars.
While I certainly understand those who are doubtful of Nevin's conclusions I'm unable to refute the growing evidence. It seems irrational given the huge potential savings to not investigate how lower blood lead levels might impact society. If a meager investment of a few million might reap rewards measured in the trillions how can we ignore such an opportunity?
If you desire further reading on lead toxicity and it's effects here are a number of articles that are relevant. Some have already been linked in the above posting.