Researchers once did a study to understand why the French aren't as obese as Americans even though they eat food that have a relatively higher fat content. They based their study on an idea they had about internal and external cues.
Apparently, Americans are more likely to use external cues ("there is still food on my plate") and the French are more likely to use internal cues ("I don't feel hungry anymore") when it comes to deciding when to stop eating.
I know for myself, the external cue of "there is still food on my plate" is a strong one, since I was raised to always finish my plate. I now do so religiously, but try to set myself up for success by getting smaller portions :).
I also do not like to waste as I have actually seen people in some parts of the world who are malnourished and always hungry... I can't leave food on my plate for that reason and I wish many restaurants would just server me less food. I'd still pay the same price.
Apparently, Americans are more likely to use external cues ("there is still food on my plate") and the French are more likely to use internal cues ("I don't feel hungry anymore") when it comes to deciding when to stop eating.
http://vivo.cornell.edu/individual/vivo/individual20060