People will readily blame nature for sexuality, yet people almost universally want to blame nurture for differences in intelligence. Evidence from the Minnesota Study of Twins Reared Apart would disagree with such an assertion. Twins reared apart show remarkably similar personalities (corr=.5), and even more similar levels of intelligence (corr=.7), suggesting that genetics plays a much larger role in intelligence than many people are willing to accept.
Sources of human psychological differences: the Minnesota Study of Twins Reared Apart, Bouchard et al, http://www.psych.umn.edu/courses/spring05/hicksb/psy3135/bouchard_1990.pdf Accessed 2012/02/02
It stems from the very American attitude that "all men are created equal," and for that reason, people want to try their best to ignore the very fact that we are all created very different from one another. While the person of common IQ can become president (Dubya, anyone?) the person of really low IQ is certainly not one we want to be voting for; maybe "You can be whatever you want when you grow up" is not as true as people would like to believe it is. It's a fundamental American belief that people will fight to keep hold of, when science has shown us time and time again that there's only so much a person can do to overcome how smart they were born.
Numerous other studies have shown similar effects of genetics on intelligence, each more controversial than the last (the Minnesota Transracial Adoption Study comes to mind).