This is the basis of Cialdini’s principle of reciprocity – we cannot take without giving back. So when one individual posts that they have completed our study and would we mind doing theirs, what they may as well say is “hey, I’ve done your study, now do mine.” As they have already done something for us we cannot possibly say no to their request without appearing like a moocher - which nobody wants. The reciprocity principle has been investigated by Regan (1971) in his art appreciation experiment
Subjects were asked to rate the quality of paintings with a partner (actually a confederate). Halfway through the exercise, the confederate would leave the room and return a little while later. For some subjects, he would bring back a can of Coca-Cola. For others he would return empty-handed. At the end of the rating exercise the confederate asked the subjects whether they would be willing to do him a favour by purchasing raffle tickets from him. As predicted by the reciprocity principle, the subjects who had received the gift of the Coca-Cola were more likely to purchase tickets than those we didn’t receive the gift; as demonstrated by the higher values in the favour column than the no favour column in table 1. This effect was present even though the tickets were more expensive than the soda had been (at the time of the study the price of a coke was only 10 cents in the US, whilst the confederate was selling the raffle tickets at a price of 25 cents each). It is also interesting to note that whether the confederate was liked or not by the participant did not have a significant effect on the rates of compliance to the raffle ticket request if the coke had been given (as evidenced by the similar values in the pleasant and unpleasant confederate rows in table 1). Hence it appears that our desire to be reciprocal overrides any of our feelings towards the person doing the asking.
Reference: Regan, D. T. (1971). Effects of a favor and liking on compliance. Journal of Experimental Social Psychology, 7, 627-639.