How to avoid gender bias when writing letters of recommendation? Finding letter writing guidelines that simply say something like "avoid gender bias" is rather easy.  Finding letter writing guidelines that explain what this means in more detail is extremely challenging.

What do men and women tend to do differently when writing letters?  What do letter writers tend to do when writing about men or women candidates?  How can these biases be recognized and mitigated?

This study by Bell, Cole, and Floge shows that male letter writers write differently than female letter writers and that both write differently when recommending men for a position than when recommending women.  But that's one study, published in 1992, and it is relatively hard to extract concrete advice on avoiding gender bias when writing recommendation letters from this paper.  Is there other work in this area?  Can it be distilled into something that's more directly applicable to letter writing?

(Note that there is a somewhat related question about avoiding gender bias when evaluating prospective applicants for an academic position.)
When I write my letters, I try to consciously avoid the biases that have been reported in the literature. Namely, I try to avoid stressing the effort/emotion/affect of female candidates. For both genders,  I try focus on research capability and competence.

You may find these helpful:


http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2572075/
http://www.academic.umn.edu/wfc/rec%20letter%20study%202009.pdf
The graph below can be found in this paper: https://www.ncwit.org/sites/default/files/resources/avoidingunintendedgenderbiaslettersrecommendation.pdf 
the original source is: Exploring the Color of Glass: Letters of Recommendation for Female and Male Medical Faculty doi: 10.1177/0957926503014002277 where the caption reads "Semantic realms following possessives. Rank-ordered within gender sets from equal numbers of letters ‘her training’; ‘his research’."