This paper examines the effects of firm-level common ownership on the level and efficiency of investment when firms face uncertainty. There is a current debate about the costs and benefits of common ownership, whereby a firm owns large stakes in multiple companies in the same industry. Critics of common ownership argue that it reduces competition and leads to a deadweight loss for the economy through decreased investment. Proponents of common ownership suggest that it allows firms to increase

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... vestment due to a reduced threat of involuntary knowledge spillover to rivals. This study contributes to this debate by examining the effects of uncertainty on these relationships. We find that common ownership allows firms to enjoy the value of waiting in the presence of uncertainty resulting in decreased investment that is more efficient. In contrast, common ownership increases investment when uncertainty is low, but these investments are less efficient. These results indicate that common ownership provides benefits to firms in the face of high uncertainty, but also allows firms to engage in valuedestroying activities when uncertainty is low. These findings are important to regulators as they debate regulations that could limit common ownership. 1 The primary findings are robust to employing an alternative measure of common ownership proposed by Gilje et al. (2019) . Please see Section 5.2. 4 While we measure Capex at time t to remain consistent with Eisdorfer (2008), our results are robust to measuring Capex at time t+1 (i.e., the firm's year-ahead capital expenditures).