Abstract: The evaluation of the causal effects of tenure length on political dynasties is a difficult task as tenure length is potentially endogenous in a model of political dynasties. To overcome this identification problem I exploit a natural experiment in the allocation of the duration of legislators’ terms in Argentina. With the return to democracy and the formation of the Congress of 1983, the duration of elected legislators’ terms was randomly allocated. I exploit this exogenous variation for the duration of legislators’ terms as an instrument for tenure length. I find that having a longer tenure in Congress increases significantly the probability of having a relative in future congresses.