But if stores can make the most of music, smell and entertainment to ‘prime’ customers, and use exclusive offers to draw them in, just as important is adapting to how we naturally behave. Here, layout is crucial — and it’s the supermarkets who are king. Midway through our trip, Fagan and I pop into M&S. Although the aisles are tightly packed, the entrance is clutter-free — with good reason. It’s thought we take between five and 15 paces to slow our speed after stepping off the street, so shops have devised a ‘decompression zone’, a spacious area that eases us into the shopping process. We also deliberate longer over the first purchase than others, picking up ‘shopping momentum’ as we go. As a result, retailers place special offers towards the front to coax us into making that first step. Staples such as milk, bread and meat are usually at the back of the store (forcing customers to pass other tempting goods) and luxuries, such as champagne and mince pies, are saved for the end. Psychologists at Bangor University pinpointed 23 minutes as the point at which our brains stop devoting full effort to calculating the value of special deals. At 40 minutes, we begin purchasing entirely emotionally. I reach for a sample of Christmas chocolate; Fagan points out that samples, too, can get us to buy. ‘It creates reciprocity. There’s research that shows that if we’re given something, we feel indebted.’