My intuition (based on anecdotes and observations, not hard data) is that nature provides a starting point, and perhaps an ultimate cap or speed limit to growth, but nurture provides the year over year progress. The most successful people I know nailed the nurture (or hard work) part of the equation. They were all at least smart, but very few were the smartest in the room at a young age. Over time they've appeared smarter and smarter to their surrounding audience.