A new study in PLoS looks specifically at one of several possible environmental factors linked to arrest patterns in general and arrest for violent crimes in particular: Exposure to lead.
Lead is a toxic metal that damages the nervous system when ingested or inhaled. It is present throughout the environment because of its widespread use in the past in paint, solder for water pipes, and gasoline. In 1978, 13.5 million US children had a blood lead level above 10 μg/dl, the current US Centers for Disease Control and Prevention blood lead level of concern (the average US blood lead level is 2 μg/dl). Lead paint and solder were banned in 1978 and 1986, respectively, by the US federal government; leaded gasoline was finally phased out in 1996. By 2002, only 310,000 US children had a blood lead level above 10 μg/dl. However, children exposed to lower levels of lead than this–through ingesting flakes or dust residues of old lead paint, for example–can have poor intellectual development and behavioral problems including aggression.
This new study claims to address lead exposure and its possible linkage to behavior more directly than previous studies. Here, prenatal and childhood blood lead concentrations are directly linked to criminal arrest records. Pregnant women in poor areas of Cincinnati, which had a high concentration of lead sources, were recruited into the study between 1979 and 1984. Their blood lead levels were measured, and the children’s blood lead levels were monitered later on through the age of six and a half. Local court records were then correlated with these data to measure …
… how many times each of the 250 offspring had been arrested between becoming 18 years old and the end of October 2005. The researchers found that increased blood lead levels before birth and during early childhood were associated with higher rates of arrest for any reason and for violent crimes. For example, for every 5 μg/dl increase in blood lead levels at six years of age, the risk of being arrested for a violent crime as a young adult increased by almost 50% (the “relative risk” was 1.48).
The results of this study may be a good example of the disconnect between the term “significant” in day to day language and “significant” statistically. A statistically significant association was found between various exposure levels and various arrest rates for individuals in the study. However, the size of the effect is not overwhelming. The largest effect was actually sex (males had several hundred times more likelihood of being arrested than females) and most other studies linking risk factors and behavior, in my opinion, show stronger effects than this study showed for lead exposure. The amount of variation in lead exposure across the individuals in this study looks pretty large. The amount of variation in arrest patterns per sex is not large.
In other words, if you created a set of identical parallel universes, and in each one removed one risk factor (parental violence, level of education, sex of subject, nutritional qualities, and lead exposure) from each parallel universe and looked for changes in criminal rate, you would not notice the difference in the case where you removed the lead.
Moreover, it is necessary to understand that this study was conducted in a fairly typical American Urban ghetto (mainly African American in this case). Virtually every subject was arrested at one point or another. In an environment such as this one, police are assigned to round up all individuals who are outside on a particular evening several times during summer months (that is one way that everyone gets arrested) and it is assumed that groups of youths congregated in any public location are committing crimes, and they are often arrested as well. A simple change in police practice would change arrest rates dramatically. In a context like this, it is very difficult to measure actual behavioral effects unless they are profound.
I do not mean to suggest that lead poisoning is unimportant! Nor do I mean to suggest that this study is flawed. What I am saying, simply, is that lead exposure may well be a factor, as previously thought, and this study confirms this better data than previously used. But, we should be careful to not assume that a magic bullet cure for a set of social ills has been found.
The authors note that over time there has been a roughly parallel decrease in lead exposure and crime in US cities. They also note that the actual casual chain linking lead exposure to arrest is not clear.
Wright, J.P., Dietrich, K.N., Ris, M.D., Hornung, R.W., Wessel, S.D., Lanphear, B.P., Ho, M., Rae, M.N., Balmes, J. (2008). Association of Prenatal and Childhood Blood Lead Concentrations with Criminal Arrests in Early Adulthood. PLoS Medicine, 5(5), e101. DOI: 10.1371/journal.pmed.0050101