Before noting the studies, I should note that I am very skeptical about what it is that IQ tests measure, and whether they are measuring it accurately. Personally, I find the thesis that there are many types of intelligence to be more persuasive, but then I do not hold my self out as an expert on measuring intelligence. For a quick summary of alternative views of intelligence, see http://www.aboutintelligence.co.uk/what-intelligence.html.
Babies exposed to pesticides before birth may have significantly lower intelligence scores by age 7 than children who were not exposed, according to three separate studies published within the last 2 weeks. In one study, a team of researchers found that every tenfold increase in prenatal exposure to organophosphate pesticides corresponded with a 5.5 point drop in overall IQ scores in children by age 7. In the other two studies, the researchers also looked at prenatal exposure and measured IQ at age 7. One group of researchers sampled pesticide residues in maternal urine, while the other tested umbilical cord blood levels of chlorpyrifos, part of a class of pesticides known as organophosphates that are known to be toxic to brain cells; it was widely used for residential pesticide applications until banned by EPA in 2001 (it is still used for agricultural applications). While the methodology of each group was different, they all determined that there was a reduction in IQ.
These studies add to the body of studies that have found a variety of adverse neurological and cognitive effects upon humans from pesticides, particularly among those exposed prenatally or at a very young age.
The three studies can be found at http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.1003185, http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.1003183, and http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.1003160. Earlier studies on the same and related topics can be found at http://ehp03.niehs.nih.gov/article/info%3Adoi%2F10.1289%2Fehp.1002056, http://www.environmentalhealthnews.org/ehs/newscience/pesticides-found-in-one-fifth-of-produce-kids-eat, and http://www.google.com/url?sa=t&source=web&cd=20&ved=0CFUQFjAJOAo&url=http%3A%2F%2Fwww.fwpp.org%2Fmedia%2F%3Fid%3D14&rct=j&q=pesticides%20IN%20%22Environmental%20Health%20Perspectives%22&ei=OQC9TZPTFoe6sQPwjIHRBQ&usg=AFQjCNGrsJFLGYpnU1-52lqVxzI-z-Qe5w. The latter is a synopsis of a multitude of studies and where they can be found.