Chlorpyrifos exposure had a larger association with working memory scores in the boys, who averaged three points lower than the girls with similar exposures, the study found.

“There are adverse effects overall, but you see that the effects are bigger in boys,” Rauh said. “The notion is that boys might be more vulnerable, for whatever reason.”

The researchers also looked to see if the parents could make up for the memory deficiencies. When the kids were 3 years old, the researchers measured the mom’s attentiveness, displays of affection, ability to control her negative reactions and other parenting skills. The study found “in terms of working memory, males benefit more from a nurturing environment than females,” the researchers wrote.

Rauh said the findings suggest that boys seemed to be “modestly” more influenced by environmental factors, whether chemical (chlorpyrifos) or social (the mother).

“It suggests that there may be some room for intervention,” Rauh said. “There’s a little wiggle room there, and that’s the good news.”

Why chlorpyrifos might affect boys more than girls is not fully understood, but a 2012 study of rats found that the pesticide reduced testosterone, which has a critical role in male brain development.

The new finding is consistent with what is known about how other chemicals affect boys more than girls, said David Carpenter, director of the Institute for Health and the Environment at the University at Albany.

“There’s such a variety of different chemicals that all do the same thing,” Carpenter said. “They reduce IQ, they appear to shorten the attention span and reduce the ability to deal with frustration.”

Lead, for example, seems to cause a greater IQ deficit in boys than girls, and some evidence suggests that polychlorinated biphenyls, PCBs, may have similar consequences, Carpenter said.

Previous research has shown that low to moderate exposure to chlorpyrifos during pregnancy can lead to irreversible changes in a child’s brain. According to a 2012 study of the New York City children, magnetic resonance imaging of 40 children, from about 6 to 11 years old, found that those with high exposures had more abnormalities in regions of the brain associated with memory. They also were significantly more likely than children exposed to low levels to experience attention problems and delays in cognitive and motor skills [PDF].

The new study is the first to measure chlorpyrifos in umbilical cord blood, which unequivocally shows if a mother and her fetus were exposed. Researchers do not know how those kids’ exposures, which occurred between 1998 and 2006, compare with levels in kids today because there are no data for comparison.
Dana Barr of Emory University, an expert in chemical exposure and a co-author of the new study, said chlorpyrifos exposure in the New York City children apparently has declined. The researchers did not detect it in the umbilical cord blood of babies enrolled in the study after the 2001 ban.

Data from California, which keeps detailed records of annual pesticide use, show that chlorpyrifos use on farm fields decreased 23 percent between 2001 and 2010. It is sprayed on corn, cotton, citrus and nut trees, alfalfa, grapes and other crops. Organophosphate exposure has been shown to have similar effects on children from farmworker families. In California’s Salinas Valley, Latino children whose mothers had the highest exposures to organophosphates, including chlorpyrifos, had a 7-point drop in IQ compared with children of moms with the lowest exposures.

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