By Aara Ramesh
The U.S. is currently in the grip of what experts are calling a historic crime wave, with violent crime — and homicide rates particularly — reaching levels previously seen in the 1980s and 1990s. In fact, over the last two and a half decades and until this year, crime rates have actually steadily declined in the U.S., though most experts cannot seem to agree why.
One interesting theory posited in the early 2010s chalked this phenomenon up to lead poisoning. In this piece, we take a look at that theory and what the scientific community thinks its merits are.
The lead–crime hypothesis is as follows: Lead was a commonplace ingredient in many consumer goods before the 1970s, particularly in paint and gasoline. The economic boom post World War II saw more houses built and more cars manufactured and bought, meaning that a whole generation of children was exposed to high quantities of lead. In the mid-1970s, cars began to use unleaded gasoline. In 1978, the federal government banned lead from being used in consumer goods. The gradual phasing out of lead and the lower levels of poisoning found in children from the 1970s on is what proponents of this theory say caused violent crime rates to halve over the 1990s and early 2000s.
But why does this matter? What makes lead so harmful?
According to experts, a child exposed to elevated levels of lead can suffer “lasting problems with growth and development.” The growth impairment can be both physiological and psychological, and can even affect hearing and the child’s behavior. Per the Centers for Disease Control and Prevention (CDC), “even low levels of lead in blood have been shown to affect learning and academic achievement.” Another effect of lead poisoning is that it can reduce children’s attention span. The neural damage caused by lead poisoning cannot be reversed, though some of the physical effects can be.
One study found that a prolonged or high exposure to lead in childhood could permanently damage that part of the brain that controls and regulates aggression, emotional responses, and impulse control, among other things, all of which generally culminate in bad decision-making. That, some theorists say, is pretty much “the profile of a violent young offender.”
Children can be particularly affected because their bodies tend to absorb lead at a higher rate and because brains develop quickly in early childhood. Lead poisoning need not occur only through prolonged exposure to the substance either. Sudden exposure to a large amount can also be detrimental. Lead can be found in many everyday substances, including paint, air, water, soil, food, and manufactured goods.
The degree to which lead exposure can explain crime rates varies, ranging from 0% to over 35%. One expert even says that the lead–crime theory can account for up to 90% of the trends noted in crime rates in the 20th century and across the world.
Of course, it is important to keep in mind that lead poisoning is not the sole determinant or cause of criminality. There are many other factors that can influence the crime rate. For some, the graph patterns across decades, cities, and countries are too similar to be coincidental.
However, across most Western countries, a BBC article says, “all the broad measures of key crimes have been falling” since the 1990s. It’s hard to track the common factor between all these nations — some increased their incarceration rates, others decreased it, but almost all saw a decline in violent crime. What they do have in common is — you guessed it — lead exposure levels.
After the theory initially began to make the rounds, proponents predicted that countries that up until that point had not banned lead use would see a reduction in crime rates roughly two decades after they did so. Sure enough, Britain banned leaded gasoline/petrol later than the U.S. and Canada did. Subsequently, the rate of violent crime in Britain dropped later than it did in North America.
According to the BBC, “Lead theorists say that data they’ve collated and calculated from each nation shows the same 20-year trend – the sooner lead is removed from the environment, the sooner crime will begin to fall.”
On the other side of the debate, detractors say that just because the graphs match up well, doesn’t mean that lead exposure and crime are related — after all, the age-old scientific tenet is that correlation does not equal causation.
In addition, there has long been a pattern of scientists trying to find a biological or physical cause for criminality, rather than accepting that the root cause may be structural, social, political, or — more likely — a mixture of numerous interconnected factors.
Of course, the lead–crime theory is difficult to really put to the test and prove. The kind of rigorous and repeated scientific study needed for that would require purposely poisoning thousands of children with lead, which is obviously unethical, inhumane, and illegal.
However, over recent months, both lead levels and violent crime have been quite prominent in the news, so it seems that a reconsideration of this theory is timely. The measured conclusion experts seem to reach is that lead exposure can and likely does affect criminal behavior, but it is still not (and may never be) clear how strong that link is and whether it is sufficient to explain broad-based crime rates.
In October, the CDC announced that it was revising its blood lead reference value (BLRV) for children between 1–5 years old, lowering the threshold to 3.5 micrograms per deciliter of blood from the previous 5. Health and Human Services Secretary Xavier Becerra noted at the time that “no level of lead is safe” for children and that the effects of being exposed to lead are “toxic and irreversible.”
Lead exposure in children has trended downward since its peak a few decades ago, but it is still “a significant public health concern” according to the CDC, especially in poorer communities that tend to be populated by non-Hispanic Black people or African Americans.
A 2020 study found that around 1.23 million American children have lead poisoning according to the CDC’s older threshold. With the revision, however, that number is expected to double.