The persuasive influence of supermarket employees is all too common these days. A while ago I drove to Sainsbury's with my housemates to do our weekly food shop. I was casually browsing the isles for what I needed when I came across a stand, caught sight of a broadly smiling middle-aged woman with twenty or so small pieces of bread smothered in 'Philadelphia' cheese (the new chocolate version). Now, I can't resist a bit of chocolate at the best of times, and seeing as it was quite clearly free, and the assistant was so nice I tried some. The food was nice, but nothing I would usually buy. When I returned to the stand to dispose of my tiny paper plate I hit a problem; I felt so bad that I had taken the free sample and was about to walk away, that I decided I would buy some of the product (probably just to make the salesperson happy).
My dilemma here is a classic example of the 'reciprocity principle'. Regan (1971) performed an experiment designed to discover whether people were more likely to buy raffle tickets from a person, if that person had previously done them a favour. In the experiment, a confederate either gave a coke to the participant, or did not. After, the confederate asked the participant to buy some raffle tickets. The compliance of the participant was measured by the number of raffle tickets bought. Regan found that participants bought more tickets when the confederate did them a favour (bought coke) than when they did not. They also found that the favour effects compliance due to the participant feeling obligated to return the favour, as opposed to increased liking of the confederate (exactly how I felt when I took the Philadelphia).
Regan, D. T. (1971). Effects of favour and liking on compliance. Journal of Experimental Social Psychology, 7, 627-639.