Like those signs in supermarkets that say ‘Limit of 12 cans of soup per customer’. They aren’t designed to ward off soup fiends from buying up all the stock, as you might think. They exist to subtly manipulate your perception of how many cans of soup you need. The brain anchors with the number 12 and adjusts downwards. One study back in the 1990s showed that precisely such a sign could increase the average sale per customer from 3.3 tins of soup to 7.52
