From the brief summary above and in the OP, it's not clear why someone would tamper with the data in this sort of study. Were I the researcher, it really wouldn't matter to me if people were more honest if they attested to their honesty at the top or the bottom of the paperwork.
I will say this sort of "study" doesn't sound like its the kind of work worthy of Ph.D. level researchers - from the summary presented here, it's more on the level of a grammar school science fair project.