In 1947 a copywriter at N.W. Ayer & Son in Philadelphia named Frances Gerety was working on the De Beers account and had gone to bed without finishing the line. She wrote it half asleep: A Diamond Is Forever. She reportedly thought little of it. Advertising Age later named it the slogan of the twentieth century, and it has appeared on every De Beers advertisement since.
The commercial context is the part usually left out. De Beers controlled the supply of diamonds and needed demand, in a market where the fundamental problem is that diamonds are neither rare nor useful, and where a resale market would have exposed both facts immediately. The Ayer campaign, running from 1938, set out to make the diamond engagement ring a social expectation in the United States, then in Japan from the 1960s, where no such tradition existed and where the campaign took the practice from near zero to a substantial majority of engagements within about fifteen years.
The slogan does two things at once, and they are worth separating.
The first is a semantic bind. It fuses a mineral with a duration and, through the duration, with the emotion. If a diamond is forever and love is forever, then the diamond is the measure of the love — which makes the size of the purchase a statement about the size of the feeling, and the absence of a purchase a statement of a different kind. The Ayer campaign made this explicit with guidance on how much should be spent, first one month's salary, later two, and in Japan three. No such figure has any basis in anything.
The second is the resale problem, which the same four words solve. If a diamond is forever, you do not sell it. Which means the second-hand market never develops, which means the retail price is never tested against what anyone will actually pay for a used stone — a number that would be, and is, dramatically lower.
The psychology is chapter 4's commitment principle and Wicklund and Gollwitzer's symbolic self-completion theory, in which people acquire and display objects to signal an identity they feel is incomplete. The ring is a public, durable, expensive artifact that certifies a private state to third parties, and it is worn on a finger where everyone can see it.
And then repetition did the rest, over eight decades, until the invented norm acquired the fluency of tradition. Ask most people how long the diamond engagement ring has been customary and the answer will be a very long time. It is younger than the aeroplane.
That is the transferable observation, and it is the bridge into Part 9 of this guide. The most successful advertising does not sell a product against competitors. It installs a norm, and the norm then does the selling in perpetuity, at no further cost, enforced by everyone who has already complied. A person who spent two months' salary on a ring is not a neutral party to the question of whether the custom is real.
Which is chapter 13, arriving eighty years later with a marketing budget behind it.
Cross-ref: Part 9 advertising manipulation