On the optimality of the patent renewal system

The patent system is mainly a renewal system: the patent life is chosen by the patentee in return for fees. I ask whether such a system cart be justif ied by asymmetric information on costs and benefits of research. In such a model I show that renewal mechanisms (possibly with subsidies) are equivale nt to direct revelation mechanisms and therefore cannot be improved on, reg ardless of the objective function. Under plausible circumstances, patents s hould have a uniform life, rather than varying in length, as typically occu rs under a renewal system.