[xenadu02]

Simple fix:

"We're conducting a scientific study on mood for the next three months. Would you mind participating? The study won't require you to do anything special or take any further action. Y/N?"

There we go kids, easy as pie; it's called consent to participate! Pretty easy to do without spilling the beans on the study.

[chris_wot]

This is a wider issue, but how do they prevent knowledge of the experiment adversely affecting the results?