HHMI has a resource on writing reference letters at this web address (a pdf)  https://www.hhmi.org/sites/default/files/Educational%20Materials/Lab%20Management/letter.pdf

Below is an excerpt on avoiding gender bias in reference letter writing from this resource. The concepts can apply to biases in general.

Be Aware of Gender Bias

Choose Your Words Wisely

Two studies have shown that there may be some differences in the choice of words used in letters for male and female scientists. Both Trix and Psenka (2003) and Schmader, Whitehead, and Wysocki (2007) found that letters for women did not contain as many “standout” adjectives—words like “superb,” “outstanding,” and “excellent”—as did letters for men. In addition, Trix and Psenka found that letters for women had more doubt-raising statements, including negative or unexplained comments.

If you say that Chris works well in a team—that might be interpreted differently depending on whether Chris is male or female. Exactly the same language can convey different messages because of people’s antecedent beliefs about gender differences. The reader might infer that Chris the man is a good team leader or someone who can work well with a team even when he isn’t a leader of it. Chris the woman, in contrast, may look as if she can’t or doesn’t lead. For both men and women, then, it is good to be explicit that the person is both a good team leader and can work collaboratively.”
—Virginia Valian, Hunter College

Test for Possible Bias

Although it is not clear what the effect of the choice of words is, it seems plausible that the words you use will affect the reader’s perception of the candidate— the stronger the words, the stronger the candidate. In an exercise conducted at the 2005 BWF-HHMI Course in Laboratory Management, course participants were asked to read two letters written by the same professor for two postdocs in his lab, one male and one female (see appendix), with similar qualifications and achievements. The professor used more specific and descriptive language, as well as more standout adjectives, for the male candidate than for the female candidate (see summary document, appendix). As a result, course participants overwhelmingly gave the male postdoc a higher ranking than the female postdoc.

Tip

Write a letter of recommendation for any candidate, male or female, and then switch all the pronouns to the opposite gender. Read the letter over and ask yourself: Does it sound odd? If it does, you should probably change the terms used.