Robert Cialdini's best-selling textbook Influence is nearly four decades old, but its teachings are as fresh anything you'd find at the latest TED.
You're feeling the power of reciprocation whenever you feel the urge to donate in response to a "lumpy mailer" from a nonprofit (one of those direct mail pieces that includes a pen, a package of seeds, or a pad of address labels).
Reciprocation—a sense of duty to repay favors in kind—rules human behavior and sets us apart from other animals.
Reciprocation explains why free samples spur purchasing; why international relief efforts promote alliances; and why concessions make negotiation possible.
Want to persuade someone? Do him a favor.
