Reference: I stole this from Paul Hebert on Fist Full of Talent.
No one is honest when there is a punishment for honesty.
Ask an employee if the incentive program drives her performance and she'll say no. Why? Because she doesn't want to admit (or believe) that she is influenced by a small reward. Then, ask she ever checks her score against her colleagues. Of course she does. They all do.
The lesson? It hurts to tell the truth. So, to find out what people are thinking, you can't ask them directly. Instead, you have to ask about their very specific behaviours.
When they can't see where the question is leading, they'll tell you the truth.