Free riders can exploit and therefore subvert joint enterprises. Empirical and theoretical research on mutual aid games indicates that the threat of punishment can curb free-riding. Since punishment is often costly, this rises an issue of second-order free-riding: indeed, the sanctioning system is itself a public good which can be exploited. Using evolutionary game theory, one can show that coercion, both by peers and by institutions, can emerge more easily if the joint enterprise is voluntary, rather than compulsory. This sheds some light on Rousseau's opening sentence of his Social Contract: 'Man is born free, and everywhere men are in chains.'