It's interesting that Paul believes willfulness is primarily inborn, especially since he now has a child. How would he react if his child were to exhibit lack of willfulness (a trait he clearly admires)?
I agree that among siblings in the same family, one often observes a wide range of willfulness. However, the same family may not be as homogeneous an environment as Paul assumes. It's been shown that elite universities disproportionately admit eldest children and only children. It would seem that the differential upbringing that only and eldest children receive (in undivided parental time and attention, etc.) does result in increased willfulness, discipline, and/or intelligence.
Ambitious people often hope that their children will exceed their own levels of achievement. Empirically, it seems that there is a strong regression-toward-the-mean effect among the children of parents who have done great things. I wonder how much of this is nature versus nurture - could there be a negative effect of growing up in too rich of an environment that diminishes one's chances of becoming a Steve Jobs or Paul Graham? How do you train willfulness and ambition in a child?