Why We Buy: The Science of Shopping

Adjacencies are related products that affect each other’s sales, as “placing one item next to another creates some spark and sells more of one or even both” (214). A toy store might put books about a favorite children’s fictional hero next to action figures of that character; some grocery stores will place a rack of bananas in the cereal aisle; often you can find paper cups in the soft drink section. A simple adjacency is a candy display at the checkout stand, where a line of customers will be tempted to buy sweets on impulse. Most adjacencies, however, are harder to achieve than they look. Retailers shouldn’t, for instance, put condolence cards next to bawdy birthday cards; in a lingerie store, don’t make the mistake of locating chairs for waiting husbands next to the Wonderbra display. 

The butt-brush factor arises from the fact that “women have an actual aversion to examining anything much below waist level, for fear of being jostled from the rear” (126). Basically, women don’t like to bend over in public. Products aimed at women therefore should not be placed on bottom shelves.