One of the more interesting trends in retail is the use of referral coupons — coupons that you can use yourself and also share with your friends. And why not? It looks like a win for everyone: The customer wins, their friends win, and the company gets more business. But according to Wharton marketing professor Raghuram Iyengar, using referral coupons is not quite as simple as businesses might expect it to be: You have to pick your brand ambassadors more carefully. He recently spoke with Knowledge at Wharton about what his research into the subject revealed about how customers react to this type of marketing. His paper on the topic, coauthored with Cornell University management professor Young-Hoon Park, is titled, “ The Impact of Referral Coupons on Customer Behavior and Firm Revenues: Evidence from Field Experiments .” An edited version of the interview appears below.     Knowledge at Wharton: Can you give us a brief overview of your research?…