Humans are cold utility maximisers indeed

At least when it comes to dining out:

When agents are ascribed selfish motives, economic theory points to grave inefficiencies resulting from externalities. In recent years, however, a wealth of laboratory evidence has questioned the descriptive validity of the selfish agent assumption.

We study a restaurant setting in which groups of diners are faced with different ways of paying the bill. The two main manipulations are splitting the bill between the diners and having each pay individually. We find that, in line with the classical model’s prediction, subjects consume more when the cost is split, resulting in a substantial loss of efficiency. Moreover, when asked to choose, diners prefer the individual pay to the inefficient split-bill method.

The paper is here. Economist's View links to an interview with one of the authors at Gelf Magazine.