At Pinterest, the online bulletin board service that is valued near $3.8 billion, some 70 percent of the users are female. But the company’s board of directors is 100 percent male.

Male-heavy boards dominate in the start-up mecca of Silicon Valley, which prides itself on progressive thinking and putting talent first. A Reuters survey of the 10 top venture-backed start-ups, as measured by venture funds raised, shows that six do not have any women on the board, including Pinterest. And none has more than one.

Reuters’ research relied on publicly available data and discussions with start-up executives and board members.

The gender imbalance has been the norm for years despite some recent signs of change. Google, Facebook and Twitter all went public without a woman on the board. They are more diverse now.

Big, established companies, by contrast, frequently have two or more female directors, based on the 10 largest U.S. tech companies by market value. All of the top 20 have at least one.

The dismal record of start-ups when it comes to gender diversity was highlighted last month when Twitter came under fire for its all-male board on the eve of its public offering. On Thursday, the company announced that it had added former Pearson chief Marjorie Scardino to its board.

Entrepreneurs and executives contacted by Reuters did not question the conclusion that there are few women directors at start-ups, but they frequently described it as unintended, and some such as Pinterest say their executive ranks are more balanced.

Start-ups tend to blame the lack of women on their boards on factors such as their youth, their small boards, their single-minded focus on growth to the exclusion of other priorities, and a scarcity of women steeped in technology. They also blame venture capitalists, who are usually male, usually hold the bulk of board seats—and don’t want to see their voting power diluted by adding noninvestor board members.

Critics counter that entrepreneurs who pride themselves on creativity and innovation simply aren’t trying hard enough when it comes to gender diversity.

At Pinterest, the focus is on work, said Chief Executive Ben Silbermann, whose board consists of himself and two male venture capitalists. “It’s pretty heads down,” he said. If the company expands the board and a qualified woman emerges as a candidate, “we’d be open to it,” he added.

“Diversity across all dimensions, including gender, is important to our success,” a Pinterest spokesman said. The company’s leadership is roughly 40 percent female, including its heads of design, finance, sales and recruiting, he added.

Public companies think about board diversity all the time – whether it’s because they are constantly being scrutinized, or because they face public investor pressure.

“The companies that want to do it will do it. Those who don’t will not,” said Maria Klawe, president of Harvey Mudd College, a computer scientist and a board member at software giant Microsoft and chipmaker Broadcom. “This stuff doesn’t happen by chance.”

The issue is not simply one of social equity. A study by the Credit Suisse Research Institute, a bank-funded group, looked at 2,360 public companies and found that over six years, the share price of large-capitalization companies with at least one woman on the board outperformed companies with no women on their board by 26 percent.

For small and mid-capitalization stocks, the outperformance totaled 17 percent.

The study identified a handful of factors that could explain the better performance, including a healthier mix of leadership skills, better corporate governance, greater understanding of consumer decision-making, and access to a wider pool of talent.

Even though it’s now conventional wisdom in Silicon Valley that a lack of women in leadership and technical roles is a big problem, progress has been very slow, especially at the top.

“Meritocracy is the word you hear all the time here. Everyone can get ahead,” said Heidi Roizen, a venture capitalist at Draper Fisher Jurvetson and a former software-company founder and Apple executive. “I don’t think it’s as clean and pure as people think.”

Women board members set an important tone, said Roizen and others, signaling to employees that talented women can succeed in the organization.

The Boys Club

That matters especially at a time when skilled technology workers are in high demand, but women are not pursuing tech careers in anywhere near the numbers as men. While women receive 43 percent of U.S. bachelor’s degrees in math and 40 percent in physical sciences, they receive only around 30 percent of doctorates in those areas, according to the National Science Foundation. They receive around one-fifth of bachelor’s and doctorates in computer sciences.

Only 27 percent of entrepreneurs are women, says Ross Levine, a professor at the University of California, Berkeley, who is researching the gender gap in entrepreneurship. Only 20 percent of startups across industries have one or more female senior-level executives, according to a study of startups by Dow Jones VentureSource.

And about 89 percent of all venture capitalists are male, according to the National Venture Capital Association, directly affecting the lack of women on start-up boards.

“If my investors don’t have more women working for them there’s not much I can do to get more women on the board,” said Dave Goldberg, the chief executive of SurveyMonkey, also one of the 10 startups surveyed by Reuters. Goldberg is married to Facebook COO Sheryl Sandberg, one of the most successful female executives in Silicon Valley.

One of Goldberg’s two independent board members, former VeriSign chief financial officer Dana Evan, is female.

Roizen and other female venture investors say their industry is beginning to change. More women are starting companies, they say, and as some of those companies achieve success that will bolster the number of female VCs, since many VCs cut their teeth as company founders.

“It’s been in the last two, three, four years,” says Theresia Gouw, a VC at Accel Partners, pointing to examples such as her portfolio company Birchbox, a beauty-products subscription business launched by two women in 2010.

Indeed, a handful of women have achieved celebrity status in the start-up world, including Sandberg, Google’s head of ads and commerce Susan Wojcicki and Yahoo CEO Marissa Mayer.

High-profile female venture capitalists include Mary Meeker of Kleiner Perkins Caufield & Byers, who is on the board of payments-service Square and funding service Lending Club, and Aileen Lee of Cowboy Ventures, who has backed companies such as Dollar Shave Club and Philz Coffee.

If a company doesn’t land a female VC on its board in its early years, when most start-up boards consists of the founder and a few key investors, it acquires a built-in chance later: when it weighs going public. Both New York Stock Exchange and Nasdaq board rules, such as building audit committees, generally require bolstering board seats.

Still, many start-ups duck and weave rather than bringing a woman onto the board. Facebook dallied until after its 2011 IPO before appointing Sandberg to the board. Google added two women to its board a year after its 2004 IPO. Now three of Google’s 10 directors are women.

Some female directors of startups say they are sometimes reluctant to overtly suggest the topic of adding more women for an open board seat. That is because they know it might raise the hackles of existing board members who want to make sure the best person gets the seat, not somebody who was less qualified but female.

“People don’t want to just stuff women on these boards,” said Ann Winblad, a venture capitalist at Hummer Winblad, who says she tends to suggest women candidates without highlighting their gender as an asset.

Many people, both male and female, assume the woman is the weaker candidate, said Meiko Takayama, founder and chief executive of advocacy organization Advancing Women Executives.

“It’s based on unconscious bias,” Takayama said. “It’s not malicious.”

Roizen herself was the subject of a Columbia Business School study that asked students to evaluate her accomplishments – but with half the students seeing her name as “Howard,” not “Heidi.” The students seeing her true name thought she seemed selfish, not the type of person they wanted to hire or work for. The students who saw the name “Howard” thought he seemed likeable.

‘Cop Out’

Many young companies acknowledge the issue, often saying they go out of their way to try to hire from a relatively small pool of technical women. But critics roll their eyes when that qualification comes up.

“I personally think that’s a bit of a cop out,” said Carol Bartz, the former chief executive of Yahoo and a Cisco board member. “There are all kinds of expertise that you need on a board. And if the entire board is technical then it is not diverse enough.”

“They’re more comfortable with their VC boys’ network and really don’t want to rock the boat,” she added.

Bartz said that sometimes the companies can make the specifications – or ‘spec’ as it is known in the recruiting industry – for the recruitment of board members so narrow that it automatically eliminates women from the candidate pool.

That spec always sounds the same, said Martha Josephson, Silicon Valley-based partner with recruiting firm Egon Zehnder: someone from the C-suite who has experience running big parts of a company’s operations.

Even if existing board members are actively looking for a woman to bolster their ranks, unacknowledged biases can come into play, said investor and former analyst Esther Dyson, whose board seats include genealogy company 23andMe, local-organizing network Meetup, and advertising company WPP Group.

“A just-okay guy seems suitable,” she said. “But a just-okay woman doesn’t quite match the awesome-woman image they have in their heads.”

Dyson forms part of a small group of sought-after female board members for start-ups that also includes Evan, the SurveyMonkey board member, and former Macromedia CFO Betsey Nelson, whose board seats include LivingSocial.

“Can you show us a pool that has more than the same names that circulate?” Nelson said she often finds herself asking when the subject arises of ensuring that women win consideration for an open seat. Evan says she is approached at least 3-4 times a quarter to join a board.

(Additional reporting by Alexei Oreskovic. Editing by Jonathan Weber and Ken Wills)