I've read research that says you only see the genetic effects on IQ in middle classes, that the genetic effects disappear with lower socioeconomic status.
My own interpretation of other research I've read (and lost links for in a catastrophic hard drive crash) is that the biggest genetic/physical factor in IQ may, in fact, be the quality of the myelin insulation on the connections between neurons. I imagine this as being something like the insulation on the ethernet cables that connect computers.
I find something oddly comforting in this notion.
