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 Î¼g/dl, the current US Centers for Disease Control and Prevention blood lead level of concern (the average US blood lead level is 2 Î¼g/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 Î¼g/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 Î¼g/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
How did they control for a possible correlation between poverty and lead exposure? I can easily imagine that poverty can prevent people from replacing old lead-containing paint or plumbing with more modern alternatives, for instance.
Beowulff: In this case, everybody in the study was poor, so they did not need to control for it. I'm pretty sure you are right ... if they compared this area to a matched suburb, the would probably find very little lead.
The correlation = causation fallacy is a concern with this kind if study. Now if we could experiment with babies...hey, why is everyone looking at me like that?
Noel: Right. But this experiment is freakishly close to an experiment. I mean, if some kid had high lead levels, did they do anything about it or just watch to see if he got arrested later? This is not clear.
It's not just poverty that could be a confounding factor. Maybe less intelligent kids are more likely to eat paint. But I don't mean to say studies like this aren't important, just that they provide less certain information than experiments. And it drives me crazy when the media report a correlation as causation. It's often completely misleading. I didn't even think of the Tuskegee type issue you bring up!
Greg, I haven't looked at the study in detail but I think you underestimate Beowulf's criticism. Even if everyone is poor income still needs to be controlled for. Indeed, if everyone's income is less then a small income difference is a larger proportion of total income. This is a serious concern.
I'd also be concerned that families that are more careful about worrying about lead exposure levels would also do a better job raising kids (I should note that example of someone I know getting lead poisoning which I am most familiar with happened with a moderate income family (they weren't incredibly well off but by no means poor) and the family didn't have any reason to think there was any problem until it was too late to do much).
Now that I'm done with all the correlation v. causation criticisms, I'm now going to suggest a causal mechanism (or at least a directional possibility). IIRC early lead exposure is also correlated with poorer impulse control. Poorer impulse control leads to higher arrest rates in a farily obvious fashion.
One of the hundreds, nay, thousands of documents I signed at the closing when I purchased an old house (1924) was the lead-based paint disclosure. So, I wonder if the fact that the older housing common to areas in which the study was run take into account that people who live in un-repaired housing liable to have such a hazard, also have other factors in their lives which tend to higher crime rates.
Crime's a mosaic, and while I think this study is interesting, I would expect that as the housing stock is replaced with that which doesn't have lead-based paint we would see a decrease in crime. Whether or not it would be statistically significant is another question.
I'm just sayin.'
Josh: The study did included SES but did not find it statistically imortant. " ...the sample was relatively homogenous with respect to sociodemographic variables such as SES and ethnicity; thus decreasing the extent to which strong confounding factors might generate spurious associations."
Everyone: Conscious behavior related to lead exposure (avoidance) as a factor presumes that these people (any people) a) have a clue as to where the lead is and how the exposure occurs and b) have a way of seeing/tasting/feeling/sensing lead in the environment. Neither is true. Little kids chewing lead paint off the window sill is the poster-child of the movement to control lead exposure. But what about the kid playing in the back yard breathing in the dust from the garden mom keeps to ensure a supply of healthy fresh veggies?
There are urban environments where there is enough lead in the topsoil to make it economically feasible to mine it.
I suspect we're in one of those neighborhoods. At the very least, we're on a busy street, and all the lead in that gas had to go somewhere. And as much as I love our mulberry trees, I won't be sorry to replace them with something that will be less tempting to neighborhood kids. I'd have done it sooner, but many of the kids around here don't actually think food comes on plants.
"Maybe less intelligent kids are more likely to eat paint."
If it helps you at all to understand the causation
Noel, cwestyin: I was a mensa kid from an impoverished clan: we had less food in the house than we had pretty paint chips.
Paint chips provided intellectual sustenance, if only because they looked healthy, and colorful.They also kept the tastebuds active, and satisfied the urge to chew, in lieu of actual food.
And the way lead paint comes off the wall in those bubbling, brittle chips is irresistible even to an intelligent adult. How can you not pick at them?