"It was on my plate"

Ask people why they ate the amount they ate, and they'll mostly say it was how hungry they were. It feels that way from the inside. The evidence says something less flattering. To a large extent, people eat what they're given.

A well-known experiment at Pennsylvania State University had 51 men and women come in for lunch once a week for four weeks. Lunch was macaroni cheese, and they could eat as much or as little as they liked. What changed from week to week was the size of the portion put in front of them, from 500 grams up to a kilo. When they were given the largest portion, they ate 30 per cent more energy than when they were given the smallest. It made no difference whether the food came ready plated or in a dish they served themselves from. The effect didn't depend on whether they were men or women, or on their weight, or on whether they were the dieting type. It showed up in everyone.

That's one study. A Cochrane review, which is about as careful as evidence summaries get, pooled 58 studies with more than 6,600 participants. It found the same thing across the board. People consistently eat more when they're offered larger portions, larger packages or larger plates and bowls. The effect was small to moderate in size, and larger in adults than in children. The reviewers rated the evidence as moderate in quality, and they noted that many of the studies had weaknesses.

Two psychologists at the University of Pennsylvania suggested a reason. They proposed that people treat whatever unit they're given as the right amount. One plateful counts as a meal, and so does one packet or one bowl, almost whatever its size. They showed it with sweets and pretzels left out for people to help themselves. When the pieces were bigger, or the serving spoon was bigger, people took much more, although nothing stopped those with the small spoon from taking a second go. They called it unit bias.

Put this beside the last chapter. Your internal signals are faint and late. The plate, on the other hand, is a clear, present signal that's in front of you the whole time. It says, "this is what a meal is". When a faint signal and a clear one disagree, the clear one usually wins. So in practice, the person who decides how much you eat is whoever served the plate. That was you, ten minutes ago, when you were at your hungriest. Or it was a restaurant, which has its own reasons for generous portions. Or it was a manufacturer deciding how big a sharing bag should be.

I find this a cheering result, for two reasons. The first is that it isn't a flaw in you. It's how people work, all of them, including the thin ones and the disciplined ones. The second is that it moves the decision to a place where it's easy to make. It's hard to stop halfway down a full plate, working against a clear signal with a faint one. Putting a bit less on the plate at the start is no trouble at all.

This isn't rationing, and here's why. Seconds stay allowed, always. The difference is that seconds are a decision and the plate is a default. If you serve a slightly smaller first helping, the plate stops doing your deciding for you. You finish it, pause, and ask whether you're still hungry. If you are, have more, gladly. Some of the time you'll find you aren't, and you'd never have found that out from behind a full plate.

One practical detail makes a difference. The serving dish on the table is a standing invitation, and the corner of the lasagne gets eaten because it's within reach. Serve from the cooker or the counter, and let seconds cost a walk across the kitchen.