Hara hatchi bu, the Okinawan people’s habit of eating only till they are 80 percent full, is thought to be one of the secrets of their extraordinary health and longevity.

Via Scientific American

I think the real question here is how the Okinawans know they're 80 percent full. Do they ever err and eat until they're 85 percent full? Does it mix things up on the days they accidentally eat until they are only 73% full? Are children punished if they eat until they're 97% full?