A form of Double Standard with many Unfortunate Implications, mostly used in older works. Fortunately largely a Forgotten Trope these days, and very hard to find played straight in modern works. This trope holds that women are less emotionally stable than men, and thus more prone to mental illness. This, in turn, is often used as a justification for portraying women as less competent than men, and less able to handle work or intellectual life, for fear it will set them "over the edge." It may lead to female characters being babied, or their opinions not being valued. This trope has its roots in the ancient Greek belief that dysfunction of the uterus could cause insanity in women. This belief was widespread up until the 17th century. In the 19th century, "hysteria" was often used as a catch-all diagnosis for mental patients who happened to be female. Hysteria is no longer used as a diagnosis by psychologists today. Compare Screaming Woman and Strawman Emotional. Contrast Closer to Earth and Emotionless Girl. This trope is about women being considered less stable, and therefore less competent and reliable than men. It is not "Any woman who shows emotion," nor is it "Any instance of The Chick, Distressed Damsel, or Faux Action Girl." Don't use this for Complaining about female characters you don't like.
Kirk: All right, Mister Spock, what do we have? A creature without form, that feeds on horror and fear, that must assume a physical shape to kill.
Spock: And I suspect preys on women because women are more easily and more deeply terrified, generating more sheer horror than the male of the species.