After Carly Fiorina was ousted from Hewlett-Packard last week, just seven female CEOs remained among Fortune 500 companies. None of them heads a Silicon Valley technology company.

The theories as to why women are poorly represented at tech companies are varied. But most pundits seem to agree – and studies back them up – that companies with women in the higher ranks are making more money, and companies that don't actively recruit and support women executives are missing the boat.

Companies with the most women in senior management had a 35 percent higher return on equity than those with the fewest, according to a study (.pdf) by Catalyst, a nonprofit group that studies women in business. It also found those companies paid their shareholders 34 percent more than companies with the fewest women in top management.

"I think almost without question that all of the companies we work with know they are able to offer better technology if they have a more diverse group of people," said Telle Whitney, president and CEO of the Anita Borg Institute, a nonprofit that promotes women in technology companies.

Some businesses are taking advantage of that. The CEOs at Lucent, Xerox and Autodesk are women, and Volvo hired an all-women engineering team to design a car. But Silicon Valley companies don't seem to get it. Another Catalyst study in 2003 found that among Fortune 500 companies, 12.4 percent of board members were women, while women represented just 9 percent of high-tech boards. The same study looked at female corporate officers and top earners, who made up 15.7 percent of all Fortune 500 employees, but just 11 percent at high-tech Fortune 500s.

"If I look ahead and think what is really going to change the world over the next decade or so, technology – not just information technology but (also) life sciences and biotechnology – are going to be profound transformational capabilities," said Carol Kovac, general manager of health care and life sciences at IBM.

Without equal participation from women, those technologies might be unappealing or simply not work for women. "You'll end up with just 50 percent of people represented, and that seems kind of dumb to me," Kovac said.

The lack of women reaches beyond Silicon Valley. The life sciences are known for attracting more women; almost 50 percent of biologists and medical school graduates are now women – but those numbers are apparently not transferring to the executive ranks in industry.

"When we look at the number of women CEOs in biotech companies, there's not that dramatic a difference from other high-tech areas," Kovac said. "So I think there are still barriers, and we need to foster an environment to encourage women, especially in technology, for their talents to shine through."

Catalyst's results show the numbers are consistently, but slowly, increasing. Reasons for the slow pace of change are unclear, but theories include the glass ceiling, an unfriendly environment toward women and the much-debunked innate gender differences theory. Whatever the true cause, it's most pronounced in Silicon Valley, experts say.

"Whatever is barring women from flourishing in computer science, engineering and information technology is the same thing that's happening in these companies," said Londa Schiebinger, a history of science professor and director of the Institute for Research on Women and Gender at Stanford University.

But there must be more to it, said Kara Helander, vice president at Catalyst for the western region, since many CEOs don't have technology backgrounds.

Suggestions about the root of the problem caused a furor among women in science recently after Harvard President Lawrence Summers suggested at a conference it could be an innate difference between men and women. Summers has apologized and said his words were taken the wrong way.

There's no evidence, and no one has apparently suggested, that Fiorina's firing had anything to do with gender. And lots of evidence indicates it had everything to do with an ill-conceived Compaq merger. But regardless of the reasons, her absence from the CEO post punctuates a dearth of female role models in Silicon Valley.

"The departure of Carly is dramatic partly because in one day we lost 12.5 percent of the women in the Fortune 500," Helander said.