Social science researchers -- going back to James Coleman's influential 1966 report ? ?Equality of Educational Opportunity?-- have long identified poverty as a strong predictor of student achievement.
Daniel Lende at the Neuroanthropology blog discusses how recent research in neuroscience is beginning to explain the damage that excessive levels of stress hormones such as cortisol have on developing brains. Impoverished homes are often stressful environments for children. Lende links to this study showing a relationship between poverty and the development of the hippocampus, a seahorse-shaped structure of the brain associated with learning and memory that is affected by stress.
Here's Lende's take on the research:
The links between poverty, inequality, stress, and brain development are no longer ideal speculation. This is robust research, even if political powers want to either ignore it or favor strategies aimed at the middle class to get votes and support early schooling as a stop-gap band-aid against the larger reality.