Think you are on to something there Mike.
If someone came up to you and said here's a tenner, take it, it's free. You wouldn't would you because no one does that unless they are wanting something in return.
One way round that is to give a plausible reason why. Here take this tenner, I've just won the lottery and I always said I would hand out some money on the street if I ever won. Would you take it then?
If you want to test scripts you need to alternate them as you go, otherwise other factors may influence results. You also need a biggish sample ie well over 100. Only change one phrase at a time, words like I'd and we are not interchangeable.