Stress

Wired has a good article on how the stress of poverty can affect the brain.

"At the same time, scientists have studied the cognitive abilities of poor children, and the neurobiological effects of stress on laboratory animals. They've found that, on average, socioeconomic status predicts a battery of key mental abilities, with deficits showing up in kindergarten and continuing through middle school. Scientists also found that hormones produced in response to stress literally wear down the brains of animals."