Why did orchestras start hiding musicians behind a screen?

Because once judges could see who was playing, the music stopped being the only thing they were judging. In a blind audition, a curtain hides the performer from the people deciding who gets the job, so an orchestra seat can be awarded on sound alone. Blind auditions have since become standard practice in symphony orchestras.

The practice grew out of a fight over discrimination. In the 1970s, the jazz bassist and clinical psychologist Art Davis sued the New York Philharmonic for racial discrimination; his lawsuit failed, but his effort has been credited with helping pave the way toward today's blind-audition system. The screen had its limits, though. In 1980, the trombonist Abbie Conant was the overwhelming first choice in a blind audition for Principal Trombonist of the Munich Philharmonic. Afterwards, the orchestra's director subjected her to years of sexism, demoting her to second chair and paying her less because she was a woman, a fight she only won after a twelve-year legal battle in which the courts ruled that his complaints against her couldn't be backed up with facts.

The most-cited evidence that the screen itself mattered came from a widely cited 2001 study by economists Cecilia Rouse and Claudia Goldin. They found that introducing blind auditions raised a woman's probability of advancing past preliminary rounds by 50 percent. Across the orchestras they studied, women went from about 10 percent of orchestra members around 1970 to about 35 percent by the mid-1990s, and the researchers attributed roughly 30 percent of that shift to blind auditions alone, while warning that their estimates had large standard errors.

The basic idea behind the curtain, hiding information that might bias a judgment until a decision is made, is the same logic behind the blinded experiments used across science: withhold anything that could sway people's judgment, and let the thing actually being tested speak for itself.