A coupon is a pricing instrument dressed as a gift. Because it takes effort to find, clip and redeem one, it sorts customers by how much trouble they will go to for a lower price, which lets a manufacturer charge two prices for the same product without ever printing two prices. Sending different coupon values to different groups also works as research: it measures how price-sensitive each group actually is. That is why the paper keeps arriving even though almost nobody uses it. Manufacturers have distributed well over 900 coupons per household a year, and The New York Times has reported redemption at only about 4 percent. The practice is old enough to be a habit rather than a novelty: Coca-Cola’s 1888 free-drink ticket is the earliest documented coupon, and between 1894 and 1913 roughly one in nine Americans got a free Coke that way, 8.5 million tickets in all. The move to promo codes, app coupons and loyalty-card discounts from the mid-1990s onward kept the mechanism and removed the scissors. The split, then, is between people who experience a coupon as money saved and people who notice it is also a nudge. More than half of Americans say they have bought something they did not need because they had one.