When charitable organizations send you a request for a donation, they almost always include a "gift" in the envelope. Sticky labels with your address on them. Or a couple of blank greeting cards. The reason they're giving you the gift is because of the social principle of reciprocity; now you will feel obliged to give something back.
You've probably heard the expression "hurry, supplies are limited!" so many times in television advertisements that it hardly registers any more. But it's there because of the principle of scarcity; your natural assumption that something that is scarce is worth more money.
These tricks, among others, are used by salespeople, marketers, and advertisers to influence people to behave in a certain way.
Every couple of years or so, I reread a great book about the psychological theories behind the science and practice of influencing the behavior of other people: Robert B. Cialdini's classic Influence. This was assigned reading in Psych 110 at Yale, and one of the most popular textbooks of the year. I assure you that every beginning car salesperson and advertising copy writer is reading this book, and you should too, if only in self-defense!
You’re reading Joel on Software, stuffed with years and years of completely raving mad articles about software development, managing software teams, designing user interfaces, running successful software companies, and rubber duckies.