Is board size an independent corporate governance mechanism?

Using a simultaneous equations framework with a comprehensive set of publicly listed Swiss companies, our findings suggest that the size of the board of directors is an independent corporate governance mechanism. This implies that any potential relationship between board size and firm valuation is indeed causal. However, in contrast to previous studies, we do not uncover a significant relationship between board size and firm valuation, which can be interpreted as support for the hypothesis of the existence of an optimal board size. On average, firms choose the number of board members just optimally. This indicates that cross-sectional variations in board size to a large extent reflect differences in firms' underlying environment, and not mistaken choices.