The board of Hewlett-Packard (NYSE: HPQ - News) has behaved pretty eccentrically of late, but its decision last week to make Mark Hurd chairman of its board of directors as well as chief executive takes the biscuit.

Hello? If any company has proof that chairing a board is an important job in itself and not merely a nice additional title for a chief executive, it is HP. In January, Mr Hurd will inherit the job of trying to broker stability within the boardroom from Patricia Dunn, who bungled its inquiry into board leaks. He will have to combine that with overseeing HP's strategy and operations.

Imagine what would have happened if Mr Hurd had been chairman when it emerged that private investigators pretended to be directors in order to gain access to phone records. The chance that Mr Hurd, a well-regarded manager, would have to resign could have spooked investors. As it was, HP's share price hardly shifted: investors thought the shenanigans were unlikely to hurt him.

Combining the jobs of chairman and chief executive is still the American way: only 7 per cent of S&P 500 companies split the roles, according to Institutional Shareholder Services. Even when the jobs are split – as has occurred at Ford between Bill Ford and Alan Mulally – it is often done dysfunctionally. A chief executive ascends to the chairmanship, but still insists on keeping at least one hand on the strategy tiller.

The implication of one person holding both jobs, or of two vying for executive control, is that the role of chairman of the board is insufficient in itself to keep an alpha male (or female) fully occupied. Founders of technology companies, such as Bill Gates of Microsoft or Michael Dell, often become chairmen but carry on opining on strategy. In Mr Gates' case, he took the additional title of chief software architect.

That kind of thing is dying out on the other side of the Atlantic. Most British companies split the roles of chief executive and non-executive chairman. In 2003, after an inquiry by Sir Derek Higgs, the combined code on corporate governance was revised to discourage chief executives who want a change of scene from simply moving up to become chairmen of the same companies.

So the US is now, as in other matters, a world unto itself. A combination of crowd psychology and vanity is probably to blame. No one wants to turn up at the golf club as a mere chief executive, no matter how well-rewarded his job and big his company, to be surrounded by two-title guys. If the chairmanship of the board is going spare, most chief executives will grab it.

Ego aside, it is tempting to run the show in the boardroom as well as the executive suite. "The US system gives one chap enormous authority with no real checks or balances on him," says one chairman of a FTSE 100 company who also sits on a US corporate board. "He call the shots on what he lets the rest of the board know. In Britain, the chairman can always say: 'We'd better tell the board, even if it is not very nice news.' "

Since a chairman's most important role is to organise the hiring and firing of the chief executive, it helps to be both. When Hank McKinnell was pushed out as chief executive of Pfizer in July, he was permitted to remain chairman until February next year. No such luck for Peter Dolan, sacked as chief executive of Bristol-Myers Squibb last week by a board chaired by James Robinson.

Mr McKinnell still lost his main job, so being chairman and chief executive does not provide all the security that it used to. The New York Stock Exchange changed its listing rules in 2003 to encourage the appointment of lead independent directors. These directors chair board meetings that executives do not attend, which curbs the ability of chairmen and chief executives to stifle unrest.

Nor does a split between the jobs guarantee good governance. An over-mighty chairman may try to interfere with the running of the company, which not only makes life hard for the chief executive but leads to factionalism. Many UK chairmen were previously chief executives at other companies and some still enjoy the public limelight. "He never took operational decisions, but he kind of gave the impression that he did," says one former UK chief executive of his non-executive chairman.

But, difficult as it can be, a division between the two jobs is the best approach. There is a clear conflict of interest between leading a board that oversees a company's management and being the senior manager. To hand both roles to one person is to invite trouble. Corporate governance reforms such as the appointment of lead independent directors have helped to ameliorate the problem, but they do not obviate it.

In practical terms, it is harder than it used to be for chief executives to fulfil the chairman's duties as well. HP is probably a special case, since bringing order to its board appears to be a full-time (perhaps impossible) job. But the rise in shareholder activism and growth in corporate governance rules require plenty of time and attention from board chairmen. No chief executive who is doing his or her job has much of either to spare.

A rational chief executive might well conclude that he or she is better off without the responsibility. This makes it all the more surprising that Mr Hurd, who seems to be a sensible chap, took on the chairmanship of HP. Perhaps he could not resist the appeal of becoming a two-title guy, along with most of corporate America. But it was an error, both for him and for the company that he leads.

john.gapper@ft.com