As a general rule, if a state commits a wrongful act, it is under an obligation to make full reparation for the injury caused. Yet, arbitral tribunals have used the doctrine of contributory fault to reduce the amount of damages awarded to the investor for the state’s breach of an investment treaty. This article analyses fifteen arbitration cases in which the respondent argued that the claimant contributed to its injury, either as a result of the investor’s bad business judgment or because the investor’s behaviour provoked the state’s wrongful conduct. The analysis shows that arbitral tribunals supported their decisions to reduce the amount of compensation by as much as 50% with strikingly brief reasoning and instead relied on their wide margin of discretion. The article discusses possible avenues to remedy deficiencies in arbitral practice dealing with contributory fault, both through the reform of investor-state dispute settlement and practical solutions that may be adopted by arbitral tribunals when considering allegations of contributory fault.