Reciprocity. The unspoken rule that we should repay in kind what another person has done for us. It’s one of our most powerful tools in sales or influencing others. Does it work? Here’s one example. A university professor tested the principle sending Christmas cards to perfect strangers. He was amazed as he received numerous holiday cards from these perfect strangers who never asked how they knew him. They received his card and felt obligated to respond in kind…