"It still tastes good"

This is the most natural belief of the lot. The food is delicious. Each mouthful is a pleasure. Stopping means fewer pleasures, and carrying on means more. If the lasagne was worth eating at the first forkful, it's hard to see why it wouldn't be at the fifteenth.

The trouble is that the fifteenth forkful isn't the first one, and this has been measured. It's among the better established findings in the study of eating, and it goes by the clumsy name of sensory-specific satiety. As you eat a food, the pleasure you get from that particular food falls. Other foods, which you haven't been eating, stay about as appealing as they were.

In one experiment, 48 people were given a four-course lunch. Some got a different food at each course, and some got the same food four times. Before and after, they rated how pleasant various foods tasted. The pleasantness of whatever they'd just eaten dropped quickly, and the foods they hadn't eaten held steady. The people given four different courses ate 60 per cent more energy than the people given the same food four times, mostly in the last two courses. The variety kept resetting their pleasure, so they kept eating.

A second study looked at timing. People rated foods, ate as much as they wanted of one of them, and rated them all again at intervals over the next hour. The biggest drop in pleasure for the food they'd eaten came two minutes after they finished. It didn't build up slowly as the food was digested. The researchers concluded that it comes from the senses, from the taste and smell and texture of the food in the mouth. It doesn't depend on the stomach filling.

This explains something everyone has noticed, which is that there's always room for pudding. You couldn't face another mouthful of roast beef, and then the crumble arrives and you find you could manage some. That isn't weakness or self-deception. The beef has worn out its welcome with your senses and the crumble hasn't started yet. The same thing is going on at a buffet, where thirty dishes keep resetting the clock.

It also says something about the extra. By the time you've reached enough, you've been eating the same food for ten or fifteen minutes. The pleasure per mouthful has fallen a good way from where it began. It's like playing your favourite song six times in a row. By the sixth time you're barely hearing it, though it's the same song. You're now eating partly from the memory of how good the first mouthfuls were.

Here's the honest objection. "It doesn't fall to nothing. The fifteenth forkful is still nice." I agree. It's still nice, mildly. So the fair comparison isn't between pleasure and no pleasure. On one side is a mild, fading pleasure now. On the other is a comfortable evening, plus the same lasagne tomorrow. Tomorrow you'll be hungry again, and your senses will have recovered. The first forkful then will be as good as the first forkful today.

That last point changes the sum. Stopping at enough doesn't lose you the lasagne. The part you lose is the worst part of it, the forkfuls that were giving you the least. If you keep what's left, you get its best part a second time.

You can check this at your next meal. Notice how good the first mouthful is, and give it a mark out of ten if you like. When you think you might be at enough, take one more mouthful and mark that. I can't tell you your numbers. I'd be surprised if they matched.