"It's just a small packet"

Some readers will say the sharing bag isn't the problem, because they buy the ordinary ones, or the packet isn't that big, or a hundred and fifty grams isn't much.

Natalina Zlatevska and colleagues, in 2014, pooled the research on how portion size affects how much people eat, and produced a figure. When a portion is doubled, consumption goes up by about thirty-five per cent on average. Not doubled, because people do stop somewhere, but a third more, from the same person with the same appetite, because the container was bigger. The effect had a shape. It was strongest for snack foods and weakest for non-snack foods. It was weaker in contexts where more attention was being paid to the food. And it flattened as portions got very large, so the difference between huge and enormous matters less than the difference between a small bag and a sharing bag, which is exactly the range you're in.

The Cochrane review that an earlier book in this series uses found the same direction across seventy-two studies. Larger portions, packages and tableware raise what people eat, by a moderate amount, reliably enough that the reviewers were confident in the direction if not the exact size.

Now do the arithmetic for your cupboard. A single-serve bag of crisps is about thirty grams. A sharing bag is a hundred and fifty. That's not a doubling, it's five times, and Zlatevska's curve says the effect flattens at the top but not at the bottom, and thirty to a hundred and fifty is the bottom. The sharing bag doesn't make you eat five times as many crisps. It makes you eat far more than the small bag would have, and, because of chapter 3, it makes you eat all of them, because the bag is the unit.

"It's just a small packet" is true of the small packet. The small packet is the one this book will end up recommending you buy, in chapter 21. What's in the cupboard now is the big one, and the big one was chosen for you, in a shop, by someone who'd read the same research and drawn the opposite lesson.