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You can't judge a book by its cover.

It's an old cliche, but it's one that even the most cutting-edge tech firms might want to revisit -- at least if they mean what they say about diversifying their workforces.

On Thursday, Speak With a Geek, a technology industry recruiter founded in 1999, revealed the eye-opening results of its experiment with blind job auditions.

In blind auditions, candidates' identifying details are stripped away and employers judge on qualifications alone -- gender, race or even where applicants went to school aren't part of the hiring calculus.

On two different occasions, Speak With a Geek presented the same 5,000 candidates to the same group of employers. The first time around, details like names, experience and background were provided. Five percent selected for interviews were women.

You can guess what happened next, right? When identifying details were suppressed, that figure jumped to 54 percent.

Diversity in the tech industry has become a major issue, and tech firms from Apple to Facebook to Intel have been releasing annual reports to show how they're doing. The numbers, though, are often lackluster, with little improvement from year to year. For example, Apple's latest diversity report earlier in the month showed the company is 32 percent female, which is 1 percentage point higher than last year. Many companies are trying to pinpoint the cause, yet there are myriad ways women and minority job candidates can get derailed or overlooked.

Speak With a Geek seems to have zeroed in on one.

Removing traces of gender or race may prevent employers from basing interview decisions on a conscious or unconscious bias, Speak With a Geek said. The former would be an outright belief that women, for instance, aren't as good at coding as men are. The latter would be more subtle and might mean a company picks the man because he better fits that employer's unexamined idea of who a coder is.

Blind auditions aren't new. In the '70s, The New York Times notes, symphonies started having musicians audition behind partitions, and researchers at Harvard and Princeton found (PDF) that when blind auditions were used, the odds of a woman being hired by an orchestra jumped from 25 percent to 46 percent.

Over the years, there have been various studies, both formal and informal, dealing with the question of whether job applicants with foreign or ethnic-sounding names are more often passed up for jobs. The results often say yes.

Some companies are offering ways to conduct blind auditions. GapJumpers, for example, helps eliminate biographical information from the hiring process. Employers first get to know candidates by way of scores from a specially designed skills test. Blendoor, in beta, is a mobile app that hides candidates' names and photos.

Tech companies should be interested in diversifying, said Annie Ryan, Speak With a Geek's director of diversity and inclusion. Research has shown that diverse teams lead to diverse thinking and better problem solving.

"If you're coming up with a new technology," Ryan said, "you want to make sure it's most applicable to the widest amount of users, so that anybody in the world can pick up your technology and have it be relevant to their lives. The best way to do that is to bring in different perspectives to the creation process."