Bingo. It's a management problem, not an individual problem. Management can turn "Yes, if..." people into mindless drones over time by constantly shooting them down and feeding them negativity. And where does that negativity come from? The next level up; someone above that manager telling them they can't do things and what they should be doing instead, or rewarding them for aligning their team's goals with the status quo. More often than not, it's driven by fear and the danger of getting fired, or a strong desire to fit into an existing structure and climb the ladder.

In my opinion, most people will be "Yes, if..." people given the right cultural conditions; this looking at people as dichotomous types set-in-stone is dangerous and in my experience incorrect. It is as divisive and ineffective as the current US political climate. If someone has become a No person, it's likely because they're being moulded into that by their management, or has been in the past. Fix the environment and create an innovation-positive management structure and they'd probably jump at the opportunity to take more risks. I have seen this happen first-hand.

W. Edwards Deming, 8th point: "Drive out fear, so that everyone may work effectively for the company." http://en.wikipedia.org/wiki/W._Edwards_Deming