Corporate governance in the United Kingdom and the United States shares many similarities, to such an extent that people often refer to the "Anglo-US model." When it comes to separating the roles of the chairman and the chief executive officer, however, this model accommodates a stark difference.

In the United Kingdom, about 95 percent of all FTSE 350 companies adhere to the principle that different people should hold each of these roles. In the United States, by contrast, nearly 80 percent of S&P 500 companies combine them—a proportion that has barely changed in the past 15 years. Why are the two countries held up as having the best corporate-governance systems polar opposites in their top-leadership structures?

The view in the United Kingdom—and other countries that embrace the idea of separation (exhibit)—is that it constitutes an indispensable component of board independence because the tasks of the chairman and the CEO are different and potentially conflicting. The CEO runs the company; the chairman runs the board, one of whose functions is to monitor the CEO properly. If the chairman and the CEO are one and the same, it becomes more difficult for the board to criticize the CEO or to express...