So, from the beginning, Slack forwent whiteboard interviews in favor of a blind code review—modeled on the blind auditions that orchestras hold—in which candidates are given a problem to complete at home. All personal identifiers are wiped from each candidate’s homework, which is then evaluated against a rigorous checklist. Not only does this help eliminate stereotype threat, but it assures candidates they’ll be judged fairly. When it became clear, last year, that this approach presented additional challenges for candidates who cared for children and didn’t have dedicated time for homework, the company shifted gears again. Candidates now have the option to do the assignment in the office if they prefer. “It’s a huge competitive advantage to be empathetic,” Grace told me. “Candidates know that the company is excited to accommodate them.”

In 2016, Slack also revamped how it interviews candidates. Bias has the potential to wreak havoc on that process: Interviewers may inadvertently favor candidates who resemble themselves, and if criteria for a job are ambiguous, interviewers may mentally rejigger those criteria to fit whatever a favored candidate has. The technical term for this is “redefining merit,” and it’s a classic manifestation of bias. (In one study, people evaluating candidates for police chiefs were asked whether education or experience were more important for the job. When the male candidate had more education, they said education was more important. When the female candidate had more education, they preferred experience.)

So a team at Slack rebuilt the interview process. For each role, the team determined what characteristics and skills a successful candidate should have—communication skills, say, or capacity for teamwork. Then, for each of these, they defined what information they needed to assess those skills, and then devised a list of behavioral questions expressly aimed at sussing out that information, questions like “Tell me about a stressful situation, and how did it go?” or “Tell me about a change to your code base.” Each candidate now faces the same set of questions. “If you ask every candidate the same question,” Grace told me, “you start to see what a good answer looks like.”

Finally, interviewers were asked to do mock interviews with existing employees, the way doctors practice on fake patients. The interviewers themselves became more skilled—and less likely to introduce biases that could filter out good candidates.

This approach, in aggregate, seems to be working: Women in technical roles, for instance, are up almost 5 percentage points from last year. “Folks on my team very regularly talk about how this is the most diverse company that they've ever worked at,” Grace told me.

Slack’s numbers aren't perfect. While Slack’s technical population is 13 percent underrepresented minorities, black and Hispanic students make up 22 percent of computer science majors in the United States. Black and Hispanic women are scarce at Slack, making up only 6 percent of U.S. employees. Erica Baker, an African-American engineer who has been a vocal advocate for more inclusion in tech, said in 2016 that she was pushing Slack to publish figures on its retention rates and be more transparent about salaries, in an effort to broaden the company’s focus from simply hiring more underrepresented minorities to helping them succeed and feel included there. Baker left the company last year, implying that she experienced difficulty moving up as quickly as she would have liked. Slack’s executives are 100 percent white or Asian. Only two of its eight top executives are women. “We exist in the actual world—if we all agree that this world has some systemic issues, and it’s sexist and it’s racist—that’s not going to stop when people walk into our office,” Butterfield has said, acknowledging the work that has yet to be done. New efforts include working on curricula with Code2040 to ensure workplace success for its trainees, and developing ally training with the Transgender Law Project.