In a group ordering situation, there is never enough pepperoni pizza.
This is because we order with our heads, but we choose slices with our hearts.
In a voting situation, the person who votes for pepperoni will consume almost exclusively pepperoni slices. But the person who votes for Vegan Veggie Lover’s Self-Flagellating Delight will probably also grab a slice of pepperoni.
Stu’s Pizza Constant = 1.75. Estimate how much pepperoni pizza you require, then multiply by this number.
See also: Stumbling on Happiness by Daniel Gilbert. This excellent book contains revealing studies about how we are terrible at predicting what will make us happy.