Existing literature on the evaluation of the economic consequences of board reforms has some limitations including: their estimation results fail to show the causal effects of the regulatory reforms; they have limited policy implications for an economy where family businesses are dominant; and there is a lack of consensus on the impacts of the reforms. Using a unique dataset and program evaluation methodologies, this paper investigated the performance of share prices in response to the newly introduced outside director system in Korea, where family businesses have been dominant. First, we find that the positive impact of the appointment of outside directors on returns is observed only when the proportion of outsiders on a board increases significantly and their appointment is accompanied by the lead-and-lag effect of regulatory reforms. Second, the buy-and-hold abnormal return is more evident for independent firms than for chaebol affiliates, due partly to the high monitoring costs for cross-shareholdings among affiliates.