>> a double portion of pasta? >> the next group. >> a half portion of pasta. >> a half portion of pasta. >> and the next. >> a full-size portion of pasta. >> reporter: a good meal is had by all. and then we let them in on the secret. >> all right, forks down. you guys were just part of a little experiment. >> reporter: those who think they have regular portions, they ate it all up, except for her. she ted it was too cold. and the ones who believe they are eating the half size portion they join the clean plate club. >> where's my other half? >> reporter: the students who think they have a double portion all left food. >> double portion doesn't sound like what you should be eating. >> if you tell them they have a small portion, oh, my gosh i will finish it all and i don't have to feel guilty. >> reporter: the professor says this explains the nation's obesity problem. we eat what we are served without thinking of how much is