Does gender bias shape academic letters of rec?
Yes, which likely hurts women's chances on the job market.
If you were hiring a new professor to join a research department at a university, would you prefer someone described as thorough, or brilliant? Would you be enthusiastic about a candidate who's "unlikely to be a superstar, but [is] solid"?
In letters of recommendation, women on the academic market are more likely than men to be described in "grindstone" terms—such as diligent, thorough or hardworking—and in terms that raise doubts about their abilities.
These patterns seem to hurt women's chances on the academic job market.
When letters raise doubts about a candidate—for example, by mentioning negative traits or hedging—hiring committees evaluate the candidate less favorably. That's true regardless of a candidate's gender. But since letters for women include more doubt-raisers, they're at a disadvantage.
Assuming advisors want to promote their students, gender stereotypes probably creep into letters via unconscious bias. If that's the case, instructing letter writers to focus on candidates' research skills and avoid doubt-raisers may help close the gap.