The existence of human and also of animal societies would be impossible without restrictions on the behaviour of the members of these societies. In human societies these restrictions are expressed explicitly (for example, by laws about not stealing), they are quite generally accepted (for example, do not lie), or they are unconsciously followed (for example, keeping oneself a certain minimal distance away from other persons).
Mostly, these restrictions are implicitly contained in the models we use in decision theory. The sets of feasible actions are constructed in such a way that stealing or not paying one's taxes is impossible. But what is it that makes us accept these restrictions and what would happen if there were nothing to make us accept them?
The latter question is answered by the economics of externalities. (Stealing, for example, does influence not only the utility of the thief, but also that of the victim.) Usually, the equilibrium situation is not Pareto-optimal. Lack of all kinds of restrictions would make a society as we know it impossible:
'Where is the railway station? he asks me. 'There', I say, pointing to the post office, 'and would you please post this letter for me on the way? 'Yes', he says, determined to open the envelope and check whether it contains something valuable. (Sen, 1977, p. 332).
And what prevents us from behaving like that? Firstly, there is the fear of external punishment which is comprised of the threats of official law and of the social consequences of disregarding a generally accepted rule. There is also an internal mechanism which makes us follow certain rules. Most of us (at least I hope so) would not leave their garbage in the woods after a picnic even if nobody could see it, and most of us (I am sure) would refuse to be a hangman even for a large sum of money. Usually we call these internal mechanisms morals and I think they are more reliably in preventing us from killing and stealing and other such things than the fear of external punishment. Erkennfnis 20 (1983) 195-205.