It's Schroedinger's cat hissing at us...
The fact that we are performing measurements affects the subject, and places the subject under stress. The added attention and stress to perform will change the emotional state, greatly skew the results. So, while we will get readings, they will refer to how a person feels under duress, and will provide data that is more useful about the stress tolerance levels of the subject.
We would get inconclusive data, as the state we want to relate is is the state where there is no duress, no stress affecting the "results".