In evolutionary times, we were far more likely to die from starvation than from diseases related to obesity. In those times, when food was available, it was best to stuff oneself.
This can be seen in an experiment run by Brian Wansink.
Wansink gave a bunch of subjects some soup to eat in bowls that were affixed to tables. Half the bowls were perfectly normal. The other half, however, would "slowly and imperceptibly" refill themselves. The subjects all ate soup until they either depleted the bowls, or were full.
The group of subjects who ate from the self-refilling bowls ate 73% more soup than the group who ate from normal bowls. However, when asked later to estimate how much soup they had consumed, on average the estimates from each group were quite similar.
Moreover, each group expressed the same degree of satiety. In other words, our stomachs aren't very good at judging how much we've eaten.
Peter Herman and Deborah Mack recruited a bunch of subjects ostensibly to taste-test some food. Some of the subjects were on diets, and some were not. The subjects were all told not to eat for a few hours before the study, so they all arrived hungry.
Some were given a small milkshake to take the edge off their hunger. Others were given two large milkshakes. And still others were given no food at all.
They were then asked to sample various cookies and crackers which were provided in bowls from which they could eat as many samples as they wanted.
Not surprisingly, among the non-dieters, those who had two milkshakes ate the least (since they were already full), while those who were starving, having had no milkshake, ate the most. But among the dieters, it was all reversed. That is, the dieters who had been given two milkshakes beforehand ate significantly more than those who had been given nothing.
This is an example of what is known as the "What the hell effect". Once a dieter had had the milkshakes, that day was no longer a "dieting day", so, he figured, "What the hell? Why not enjoy myself?"
- "Bottomless Bowls: Why Visual Cues of Portion Size May Influence Intake" 2005 article in Obesity by Brian Wansink and others
- "Restrained and unrestrained eating", by Peter Herman and Deborah Mack, 2006 article in Journal of Personality
- Mindless Eating: Why We Eat More Than We Think, 2007 book by Brian Wansink
- "Willpower", 2011 book by R. F. Baumeister & J. Tierney, page 220-33