Researchers sought to estimate the effects of exposure to lead in topsoil on the cognitive ability of 5-year-olds in the United States. The study found that higher lead in topsoil significantly increases the probability that 5-year-old boys will have cognitive difficulties but does not seem to affect 5-year-old girls. The researchers found the adverse effects in boys even in U.S. counties where the government considers the levels of lead concentration in the soil to be low.