People, when eating, will stop after a certain time. "I’m full", they say, evoking an image of the stomach as a rubbish bin, which is filled to a particular point, after which you have to wait until some one puts a new bin liner in it. But that image is completely wrong: you only push your stomach to the limits in extreme cases – like Christmas dinner.

So why do people stop eating? Biologists mumble stuff about satiation molecules – which do actually exist, but are not effective when produced as appetite suppressants. Starting and stopping eating seem to be primarily a psychological matter. People who can’t store memories due to a memory disorder eat when the clock says it’s time to eat. If you put the clock back an hour, they will have lunch then. The signals to make you start and stop eating are mainly external rather than internal.

The experiment that makes this most evident was devised by American Brian Wansink, who sat his test subjects down in front of the telly with a bowl of soup. Most of them finished the soup without a fuss, but the next group of test subjects couldn’t finish all the soup: using an ingenious construction involving a pump and a rubber tube, Wansink refilled the bowls from below. The subjects just kept eating until they had consumed as much as a litre of soup.

Wansink had used the television mainly to distract the people from noticing the odd construction of the soup dishes, but this experiment dating from 2005 stresses how easy it is to eat more if you are distracted.

Concentration and eating are closely connected, or as Leiden psychologist Lotte van Dillen says: "We only have limited mental capacity but we’re exposed to an infinite amount of information. The more attention we need to spend on one thing, the less we have for another."

In a recent article in the scientific journal Psychological Science, Van Dillen discusses the matter in more detail. Test subjects were asked to memorise a number, drink a glass of either lemonade or diluted lemon juice and then write down the number. Afterwards, they were asked to describe the sweetness or sourness of the beverage. The group who had been asked to memorise a number containing seven digits described the lemonade as weaker flavoured than the group who had to memorise a one-number digit.More or less the same happened in follow-up experiments: crackers with salted butter did not seem as salty – and the test subjects ate more if they were not allowed to forget the difficult number and just as many crackers with unsalted butter were consumed. In a test in which people were asked to make their own drink of lemonade from syrup, the group who had to try and remember seven digits used more syrup than their counterparts. Van Dillen adds: "When asked how the lemonade tasted, they said it was just as sweet, but they had used as much as twice as much syrup."

In other words: if you are distracted, you have less capacity for tasting, and accordingly, you experience less flavour. That probably explains why you eat things that are actually quite revolting if you work, watch television or drive while eating or drinking: savoury snacks, sweets or energy drinks.

It may also explain why Wansink’s test subjects ate so much: it would seem that people stop eating when they have received a number of stimuli from their food. "People have specific networks in their brains for the sole purpose of enjoying food. They need the stimuli and if they do not experience the flavour very intensely, they need more food to reach the right level of pleasure. I really believe that more flavour means that you eat less."

If she is right, whole piles of diet books will become obsolete, as they always want to limit pleasure rather than encourage it. People are terrible at limiting pleasure, and diet books don’t work. Almost everyone who diets will gain weight again after a few years, usually with some additional pounds.

Another of Van Dillen’s experiments is more hopeful: in The Journal of Personality and Social Psychology, she describes how the people who had to memorise the longer number were less likely succumb to the temptation of delicious food. "You must process the stimulus before you can be tempted", she explains, "and if you’re occupied with something else, you can’t respond to the temptation."

She smiles: "But if you do succumb, don’t do anything else. Concentrate fully on your food, sample it more carefully and eat less. You need to use you cognitive capacity strategically."