From an article in Inc. Magazine by Jeff Hayden:

Nobel Prize-winning economist James Heckman likes to ask people how great a role innate intelligence plays in financial success. Like how much the difference between my income and yours, for example, is based on our relative IQs.

Most people say about 25 percent. Some go as high as 50 percent. (For a long time, I would gave guessed even more.)

But Heckman's research reveals something else entirely. Innate intelligence plays, at best, a 1 to 2 percent role in a child's future success.

Instead, financial success is correlated with conscientiousness: Self-discipline, perseverance, and diligence. [emphasis ours]