"I was hungry"

Sometimes the run gets explained by appetite. You'd had a small dinner. You were peckish. The bag went because you needed it.

Look at what the first crisp did and what the fortieth did. The first one was the taste you'd been wanting. It was good. The fifth was good. Somewhere between the tenth and the twentieth, in my experience of asking people to notice, the crisps stopped being distinguishable from each other and became a texture. By the fortieth you weren't tasting anything. You were doing something with your hand while watching a programme, and the something happened to involve crisps.

Barbara Rolls and colleagues, in 1984, gave people a four-course lunch, either the same food in every course or a different food in each, and found that people ate sixty per cent more when the courses varied, mostly in the third and fourth. As people ate a food, they rated it as less pleasant, while foods they hadn't eaten held their appeal. The researchers called it sensory-specific satiety. You get full of a taste before you get full of food. Marion Hetherington and colleagues, in 1989, timed it and found that the biggest drop in how pleasant a food seemed came about two minutes after eating it, which is fast enough to be a sensory effect and not a matter of the food reaching your stomach. An earlier book in this series uses both studies for meals. They apply to a bag with full force, because a bag is one taste, repeated.

So the pleasure of crisps, measured, is front-loaded and short. The first few are the pleasure. Within two minutes the pleasure has mostly gone, and what's left is the hand. If you were hungry, the hunger was satisfied somewhere in the first handful, and hunger is a thing that stops. The hand doesn't stop, because it isn't being driven by hunger or by taste. It's being driven by the bag being there and the screen being on, and those don't run out until the bag does.

"I was hungry" might be true of the first one. It isn't true of the last hundred. Those weren't eaten by a hungry person. They were eaten by a person who'd stopped tasting them, and the reason you feel slightly sick at the bottom isn't that you overate. It's that you ate a hundred and forty things you didn't taste and got nothing for them.