http://www.sciencedaily.com/releases/2009/08/090829092042.htm

ScienceDaily (2009-08-29) — Scientists have identified cells in the stomach that time the release of a hormone that makes animals anticipate food and eat even when they are not hungry. The finding, which has implications for the treatment of obesity, marks a landmark in the decades-long search for the timekeepers of hunger. The work reveals what the stomach “tells” the brain.

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So it’s not just willpower or environment or any one thing.  Hormones certainly play a part.