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.”