A team from the School of Family and Consumer Sciences at Eastern Illinois University have discovered a reliable method of persuading people to eat up to 50% less pistachio nuts – just leave them in their shells. Their paper on the subject : In-shell pistachio nuts reduce caloric intake compared to shelled nuts (published in the October 2011 issue of the journal Appetite) not only confirms the hypothesis that “…consuming in-shell pistachios, compared to shelled pistachios, causes individuals to consume less.” But also offers a highly plausible explanation for the effect : “The difference in calories consumed may be due to the additional time needed to shell the nuts or the extra volume perceived when consuming in-shell nuts.”
Based around the findings, one of the paper’s authors, Professor James Painter, has developed the so-called ‘Pistachio Principle’ – which he outlines here in this video 

“’The Pistachio Principle’ works because it allows us to reduce calories without restricting. Now normally we’re restricting the foods that we eat. We don’t eat doughnuts, we don’t eat fried food, we don’t eat hamburgers, we eat don’t fat, we don’t eat carbohydrates, and when we don’t eat those foods, we end up craving those foods that we’ve restricted. ‘The Pistachio Principle’ works because there is no restriction and we receive a calorie reduction without restriction.”