There was a study posted here recently that disagrees with you: https://news.ycombinator.com/item?id=10488998
The summary is that very intelligent people do not seem to have any unique genetic or environmental factors that explain that, they just happen to be at the edge of the bell curve. This is not true of very unintelligent people; there are lots of genetic and environmental factors that can decrease intelligence.
From the abstract: "We found that high intelligence is familial, heritable, and caused by the same genetic and environmental factors responsible for the normal distribution of intelligence."
For instance what accounts for the variation in play between a bunch of 7 foot tall basketball players is not what separates them from a gentleman of 5'5. Most research backs this up. The gap in achievement between an IQ of 115->130 is far large than 145->160.