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The effect is especially noticeable in kids from a California farming region where they and their mothers were also potentially exposed to pesticides through their use on local crops.

The pesticides in question are known as organophosphates, which kill insects by disrupting their brains and nervous systems. First developed in a more potent form as nerve poisons during World War II, they can disrupt people’s nervous systems as well, according to the U.S. Environmental Protection Agency.

The research doesn’t prove that organophosphate exposure is bad for the developing brains of infants and children. That may be impossible to confirm because ethical constraints prevent scientists from randomly assigning kids to be exposed to pesticides to see what happens.

Still, “there’s a body of evidence that’s beginning to build” in support of a link, said Brenda Eskenazi, co-author of one of the studies and director of the Center for Environmental Research and Children’s Health at the University of California, Berkeley.

Although organophosphates are no longer used in bug-killing products designed for the home, they remain common in agriculture, the researchers noted.

Two of the studies, conducted by researchers at Mt. Sinai Medical Center and Columbia University, examined kids in New York City, while Eskenazi’s study looked at 329 kids and their mothers in the Salinas Valley area of central California. All of the studies found links between exposure to the pesticides in pregnant mothers and lower IQ scores in their kids by age 7.

In the Salinas Valley study, researchers looked at signs of pesticide exposure in urine taken from the mothers during pregnancy and later from their children.

The researchers found that every 10-fold increase in organophosphates detected during a mother’s pregnancy corresponded to a 5.5-point drop in overall IQ scores in her children by age 7.

In fact, the 20 percent of the children whose mothers appeared to have been exposed to the least pesticides had about a 7-point higher IQ level, on average, than those in the 20 percent born to mothers with the highest exposure, the researchers reported.

The difference is equivalent to about six months of brain development in a typical child, Eskenazi said. And the differences remained even when the researchers adjusted their statistics so they wouldn’t be thrown off by factors such as the education levels of the mothers and exposure to other toxins such as lead, she noted.