This is an attempt to rescue a strongly down-voted question.
Are there any studies investigating possible unconscious gender bias in evaluating recommendation letters? Specifically, is there any published evidence that recommendation letters with female authors are (or are not) less effective than recommendation letters with male authors? Studies considering letters for graduate admission, faculty hiring, or promotion and tenure are all relevant.
Let me emphasize that I am not asking about intentional sexism, which I assume is sufficiently rare to be insignificant, but rather unconscious bias. I am also not asking for anecdotes, but pointers to actual published literature.
Similar studies have revealed significant gender disparity in several related academic contexts, including recommendation letters for male vs. female applicants. Other examples include: