Not IQ really?!

Really.

It's being yourself, having courage, being patient, having follow-through, continually learning, not giving up, and always improving:

So, what does predict success? Research tells us it’s using the right strategies that leads to accomplishment and achievement. Sounds simple, but strategies like being committed, recognizing temptations, planning ahead, monitoring your progress, persisting when the going gets tough, making an effort, and perhaps most important believing you can improve, can make all the difference between success and failure.

These things do not correlate with intelligence.

Intelligence correlates with overthinking.

Correlation is not causation. Only the intelligent can overthink, but intelligence doesn't cause overthinking.

With the right strategies, intelligence is a major boost to one's chances of success. I don't know of any specific studies that show this, but I suspect it is true. I also suspect Eric may have a study that backs me up!

IQ predicts academic and job performance quite well:

http://www.bakadesuyo.com/does-your-iq-score-really-mean-anything

However...

Self-discipline is a better predictor of success than IQ:

http://www.bakadesuyo.com/whats-better-than-iq-at-determining-who-will

Team success is best determined by how team members feel about one another:

http://www.bakadesuyo.com/whats-the-best-predictor-of-team-success-in-t

And happiness makes us successful (not just correlates):

http://www.bakadesuyo.com/does-success-make-us-happy-or-does-happiness

Much more on who ends up successful here:

http://www.bakadesuyo.com/which-people-end-up-more-successful-and-make

I summarize your findings as: Practice and happiness correlate with success.