Why is Apple's Tim Cook the only openly gay CEO of a major US firm?

It's pretty startling that among the chief executives of the 500 biggest US companies only one, Tim Cook of Apple, is openly gay.

His declaration this week that he is "proud to be gay" was hailed as an important moment for the gay community.

Campaigners say they know of bosses of other big firms who are gay, but are reluctant to be open about it.

They hope Mr Cook's public acknowledgement will spur them to come out.

So what is holding them back?

"When I was at university, people who did gay things, homosexual acts, usually went to prison and stayed there a long time," Lord Browne said in an interview with the BBC earlier this year.

He resigned as chief executive of BP in 2007, external after it emerged that statements he had made in legal documents about a four-year relationship he'd had with a man had been "untruthful".

Like Lord Browne, many chief executives would have grown up in times that were much less tolerant of gay people.

Campaigners says that formative experience has probably made senior executives reluctant to come forward.

"Societal views have changed considerably, but that has only really been in the past 10 years," says Suki Sandhu, founder of professional network Outstanding.

Gay people now in their 50s and 60s could have been keeping their sexuality a secret from colleagues for years, perhaps decades. That lengthy secrecy is another reason to maintain their silence.

"Some of our best leaders are known for their honesty and authenticity, to kind of then turn around and go, 'Actually, there's a big thing that I haven't quite mentioned', is quite difficult," says Ruth Hunt, chief executive of Stonewall, the lesbian, gay and bisexual equality organisation.

Most business people are "squeamish" about speaking about their personal lives, says Jan Gooding, group brand director of insurance giant Aviva and chair of Stonewall.

They would rather be known for their work as professionals than for their sexual orientation, she says.

But chief executives are now under pressure to reveal more about their personal lives.

"When I came out, it became clear that my life was public property and I think that when you are a leader of a large organisation, that is the case nowadays - you have to leave your privacy to one side," said Lord Browne.

Executives who do come out can expect to be a role model for younger gay staff, which can be an added pressure.

"Once you come out, you are expected to suddenly be an expert on all things gay, but of course you are an expert on your job - not all things gay," says Ms Hunt.

Chief executives could have another concern. Could coming out damage their business prospects?

The International Lesbian, Gay, Bisexual, Trans and Intersex Association lists 78 countries, external with criminal laws against sexual activity by the groups it represents.

Many of those countries are in Africa, Asia and the Middle East.

Those regions are growing fast and are where ambitious companies would be eager to expand.

Sukhi Sandhu of Outstanding thinks having a gay chief executive "could impact sales" in those countries. But that remains to be seen, as there is no data to support the theory.

After Tim Cook's announcement, we might find out over the next few years whether customers might consider the sexuality of a chief executive when choosing a mobile phone.

So what about gay people trying to work their way through the ranks in the current era?

Over three-quarters of America's 500 biggest companies have non-discrimination policies that bar firing someone because they are gay or lesbian, says Justin Nelson, co-founder and president of the US National Gay and Lesbian Chamber of Commerce.

But he says those policies can only go so far.

"There are still times that, even when you have a policy, the guy in the cubicle next to you might have a personal belief that is not fully inclusive," he says.

Some industries are working harder than others to overcome such issues.

The UK fashion industry can boast the only openly gay chief executive among FTSE 100 companies, after Christopher Bailey became chief executive of Burberry earlier this year.

Mr Sandhu says diversity is "in the DNA" of Burberry.

But the fashion industry is not necessarily leading the way in encouraging gay staff.

Stonewall chief executive Ruth Hunt says that law firms and banks are among those that have recently made the most progress recently.

The building trade is also making a big effort, she says, with construction giant Balfour Beatty working closely with Stonewall to help gay staff.

Despite the efforts of campaigning groups, many young gay people still feel under pressure to hide their sexuality.

"It's quite shocking to learn that even now graduates who are openly gay at university are going back into the closet because the business world is a little behind," says Mr Sandhu.