The first study to follow lead-exposed children from before birth into adulthood has shown that even relatively low levels of lead permanently damage the brain and are linked to higher numbers of arrests, particularly for violent crime.
Earlier studies linking lead to such problems used indirect measures of both lead and criminality, and critics have argued that socioeconomic and other factors may be responsible for the observed effects.
But by measuring blood levels of lead before birth and during the first seven years of life, then correlating the levels with arrest records and brain size, Cincinnati researchers have produced the strongest evidence yet that lead plays a major role in crime.
The researchers also found that lead exposure is a continuing problem despite the efforts of the federal government and cities to minimize exposure.
The average lead levels in the study "unfortunately are still seen in many thousands of children throughout the United States," said Philip J. Landrigan, director of the Center for Children's Health and the Environment at the Mount Sinai School of Medicine in New York.
The link between criminal behavior and lead exposure was found among even the least-contaminated children in the study, who were exposed to amounts of lead similar to what the average U.S. child is exposed to today, said Landrigan, who was not involved in the study.
"People will sometimes say, 'This is in the past. We are cleaning up lead. We don't have lead problems anymore,' " said criminologist Deborah W. Denno of Fordham University in New York, who also was not involved in the study. "The Ohio study says this is still a big problem."
Nationwide, about 310,000 children between the ages of 1 and 5 have blood lead levels above the federal guideline of 10 micrograms per deciliter, and experts suspect that many times that number have lower levels that are still dangerous.
"It is a national disgrace that so many children continue to be exposed at levels known to be neurotoxic," said neurologist David C. Bellinger of Harvard Medical School, who wrote an editorial accompanying the study published in the online journal PLoS Medicine.
Although some urban soil is still contaminated with lead from gasoline, 80% of lead exposure now comes from houses built before 1978. Paint in such houses can contain as much as 50% lead, and even if it has been covered by newer, lead-free paint, it still flakes or rubs off.
About 38 million U.S. homes, 40% of the nation's housing, still contain lead-based paint, according to the U.S. Department of Housing and Urban Development. The problem is particularly acute in urban areas, which typically have older housing that has not been renovated.
More recently, parents and authorities have become concerned about increasing levels of lead-based paint in toys imported from China.
Researchers have long known that lead exposure reduces IQ by damaging brain cells in children during their early years.
It is also known that lead increases children's distractibility, impulsiveness and restlessness and shortens their attention span, all factors considered precursors of aggressive or violent behavior.
A landmark 1990 paper by Denno linked lead to increases in criminal behavior, but the children in the study were not tested for lead levels. The diagnoses were based on their physicians' evaluation, Denno said.
The Cincinnati lead study enrolled 376 pregnant women in Cincinnati's inner city between 1979 and 1984, measuring their blood lead levels during pregnancy and the children's levels during their first seven years of life.
In the new study, environmental health researcher Kim N. Dietrich of the University of Cincinnati College of Medicine studied 250 of the original group, correlating their lead levels with adult criminal arrest records from Hamilton County, Ohio.
He and his colleagues found that 55% of the subjects (63% of males) had been arrested and that the average was five arrests between the ages of 18 and 24.
The higher the blood lead level at any time in childhood, the greater the likelihood of arrests. "The strongest association was with violent criminal activity -- murder, rape, domestic violence, assault, robbery and possession of weapons," Dietrich said.
Blood lead levels in the children ranged from 4 to 37 micrograms per deciliter.
The researchers found, for example, that every 5-microgram-per-deciliter increase in blood lead levels at age 6 was accompanied by a 50% increase in the incidence of violent crime later in life. Confirming previous findings, the effect of lead was strongest in males, who had an arrest rate 4 1/2 times that of females.
In a related study, spectroscopist Kim M. Cecil of Cincinnati Children's Hospital Medical Center and her colleagues examined a "representative sample" of 157 members of the same group using whole-brain MRI scans. They found that those with the highest blood levels of lead during childhood had the smallest brain volume.
On average, the brains of those in the study were about 1.2% smaller than normal. The most affected regions of the brain were those regulating decision making, impulse control, attention, error detection, task completion and reward-based decision making.
"The most important message is that lead affects brain volume, independent of demographic and social factors that are often used to explain away poor outcomes" in life, Cecil said. "This is independent biological evidence showing that the brain is affected by lead."