I did some research due to this debate and became pretty shocked at what I came across. From wiki (citied within):
"Estimates in the academic research of the heritability of IQ have varied from below 0.5[2] to a high of 0.9.[5] A 1996 statement by the American Psychological Association gave about .45 for children and about .75 during and after adolescence.[6] A 2004 meta-analysis of reports in Current Directions in Psychological Science gave an overall estimate of around .85 for 18-year-olds and older.[7] The New York Times Magazine has listed about three quarters as a figure held by the majority of studies.[8]"
http://en.wikipedia.org/wiki/Heritability_of_IQ
A proportion of 1 would mean ALL variation seen in the trait is genetic, so the high rates seen here are pretty startling especially with all the "work ethic, nurture over nature, etc" statements we're always told about education. I've worked with low-income kids and some honestly have tremendous difficulty with problem solving and just can't seem to put 2 and 2 together even when given all the nudges possible.