Although the promotion process might have some flaws, the flaws of the system had affected these candidates from the time they joined the firm as first-year associates. They were victims of what has become known as performance-support bias, in which some employees receive less support from the start.

Janice Fanning Madden, of the University of Pennsylvania, examined why female stockbrokers in two of the largest brokerage firms in the United States made only about 60 percent of what their male colleagues were paid. A closer look at the data revealed that this was not because female brokers performed worse than their male counterparts but because they were given worse-performing accounts to start with.

At the professional services firms, the partners also tended to look for people like themselves who, ideally, had graduated from the same university. Since the allocation of work in the first year often happens informally, these processes took place below anyone’s radar screen.

Our research shows that when people make such choices more deliberately — by explicitly comparing candidates with one another instead of focusing on just one person, for example — they are more likely to choose based on performance rather than on stereotypical associations. What’s more, variety is more likely to emerge when we make bundle decisions, that is, assign work to or promote more than one person at a time.

The associates with thin files were not only given fewer opportunities to prove themselves but had also received less feedback to help them improve. Typically, giving feedback is discretionary, with little structure provided to the supervisor. And it is hard: We do not like receiving critical feedback, nor do we enjoy giving it.

Research suggests double standards where, for example, the time taken to make a decision is interpreted as a sign of indecisiveness for a female associate but as thoughtfulness in the case of her male counterpart. Given that we hold pre-existing beliefs about the behaviors of people with certain characteristics, we tend to look for evidence that confirms these stereotypes.

Image Iris Bohnet is a behavioral economist and professor at Harvard Kennedy School. Credit... Tony Luong for The New York Times

Such confirmation bias is well documented and difficult to overcome; we often are unaware of our views or how we bring them to bear in certain situations. In addition, women and minorities have been found to get less and more vague feedback that is less helpful to development and growth. That may be because people are uncomfortable judging or criticizing members of traditionally disadvantaged groups.