This is according to the BBC anyway, who took on the challenge of trying to make sense of an industry rule. The answer, in short? It's still very much a man's world.
It's a topic that I've actually never really given much thought to to be frank (and you likely haven't either). Then again, I'm a dude, so maybe that's partly why.
But it's worth discussing: How much would hearing a woman's voice in the trailer for the next Michael Bay movie, for example, affect the audience's reactions to that trailer?
Watch the BBC report below: