Usually if you go to a restaurant with your friends, you agree to split the bill and you don't mind if you pay a little more than you really have to pay out of the whole bill.
But in this case people tend to order more expensive meals, or simply eat more than they usually do.
That can be explained by psyhology: if you pay separately, you will have to pay a lot more, but if people split the bill, you'll only have to pay a little extra.
Therefore, it's more efficient for people to share the bill, in order not to pay more, because sometimes it can also be really unfair. ( for example, if you have only had a cup of coffee and your friend have tasted 7 different meals).
