Gender bias at work is real.

A new Harvard Business Review study investigated why men are more likely to get promoted than women and found that while both genders have similar behaviors in the workplace, their bosses perceive them differently.

Researchers surveyed a business strategy firm where women were underrepresented in high levels of management. They collected emails and meeting schedule data for 500 employees in one office, across all five levels of seniority over the course of four months. Then they gave 100 people badges with sensors that allowed researchers to track in-person behavior like who talks to whom and who dominates conversations, movement, proximity to others, volume and tone of voice. They found that both genders had the same working patterns.

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And analysts discovered that women and men had the same number of mentors and had an equal amount of facetime with managers. Both genders in the same job position devoted an equal amount of time to deadlines and projects. Male and female employees also received identical scores on performance evaluations, yet it remained unclear why women weren’t getting promoted at the rate that men were within the company.

“Our analysis suggests that the difference in promotion rates between men and women in this company was due not to their behavior but to how they were treated,” the study notes.  “This indicates that arguments about changing women’s behavior — to “lean in,” for example — might miss the bigger picture: Gender inequality is due to bias, not differences in behavior.”

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HBR defines bias as two different groups of people behaving the exact same way but getting treated differently for no apparent reason. So to even the playing field when it comes to promoting employees, companies should interview candidates in groups rather than individually to get a better sense of their performance, according to a study by Iris Bohnet of Harvard Kennedy School.

Evaluating candidates up for a promotion when grouped together helped managers better compare employees strictly based on performance not gender.  When hiring managers interviewed candidates on their own, the result was bad hiring decisions, for example, more men were chosen for data driven roles.