When dining out with a group of friends, one of the easiest ways to pay is simply to divide the total check by the number of people, with each person paying the same amount, irrespective of who bought what. It's quicker, easier for the server, and reduces that awkward discussion of money over dinner.
But if you're a water and crackers guy and your friends like wine and Ahi appetizers, this obviously puts you at a disadvantage. You're subsidizing other people's expensive habits. But there's another reason not to split it like this: Your total check will also be bigger.
In his book The Economic Naturalist: In Search of Explanations for Everyday Enigmas, economist Robert H. Frank discusses this phenomenon: if a diner is choosing between a $20 dish and a $30 dish he will, without even thinking, perform a cost benefit analysis. If the the $30 dish is only worth $5 more to him than the $20 dish, he may choose to go with the $20 dish: the additional benefit does not equal the additional cost.
But if he is with a group of 5 people, that additional $10 is only $2 to him and each other person --and so it makes sense for him to go with the $30 dish, even if the added value is not equal to the total the group will pay.
I'm not saying that your friends are mooches who are consciously looking to dine lavishly at your expense. But on a subconscious level, it means that splitting the check evenly will lead to higher costs -- and more calories!
When dining out, don't split the check evenly!