What is reciprocity in marketing psychology and how does it influence consumer behavior?

Reciprocity is one of Robert Cialdini's six weapons of influence, based on our innate desire to return favors. When someone does something for us, we feel an uncomfortable urge to repay them. Phil Agnew references researcher Philip Kunz's experiment where he sent Christmas cards to 600 strangers and received 200 responses, demonstrating this principle in action. Another study by Dennis Reagan showed this effect in a controlled setting. When a paid actor bought participants a can of Coke, they were twice as likely to later purchase raffle tickets from that person compared to participants who received no drink. This psychological principle can be effectively applied in marketing to build customer loyalty and increase conversions.