Invitations

A few years ago, a university professor tried a little experiment. He sent Christmas cards to a sample of perfect strangers. Although he expected some reaction, the [blank] he received was amazing holiday cards [blank] to him came pouring back from the people who had never met nor heard of him. The great majority of those who returned a card never inquired [blank] the identity of the unknown professor. They received his holiday greeting card, and they automatically sent one in return. This study shows the action of one of the most powerful of the weapons of influence [blank] us the rule for reciprocation. The rule says that we should try to repay, in kind, what another person has provided us. If a woman does us a favor, we should do her one in return; if a man sends us a birthday present, we should remember his birthday with a gift of our own; if a couple invites us to a party, we should be [blank] to invite them to one of ours.