While some write this off as obscene misogyny — and I don’t deny that this exists — I think there are several other less insidious reasons why this scenario is a horror film cliche.
First of all, being in one’s scanties is one of the most vulnerable states for humans. Even more so for women, who live in a male dominated world where rape and other forms of misogyny exist and thin scraps of nylon fabric are not going to protect us.
Unless it is children or animals (which are often not spared from horror works either), what can be more vulnerable than a woman in her undies?
Women are also more pleasing to look at. You can make an argument for sexism here, but the curvy female form has been proven to be more appealing to both men & women than the male form. And come to think of it, no one wants to see a fat guy in panties being chased around.
Why not go further, and show her naked? Two reasons: Censors/ratings, and contrary impulses which distract.
From a business point of view, nudity is threatening. If a film shows nudity, the rating level increases, which may make the film unable to be shown to it’s target audience of teen viewers. This affects the bottom line.
From an artistic or creative point of view, nudity may not evoke the same response of vulnerability & fear. Certainly at the very least it can be distracting, with some folks uncomfortable with jiggling & wiggling. Others may find nudity arousing. Either way, you sort of break the story’s spell of horror.
So why are the girls in bras & panties, or those having sex, the first (or only ones) to die?
Frankly, I believe it’s puritanical Victorian values & the old Hayes Code rearing their ugly heads (if only Jason or Frankenstein would behead those!).
It seems that those who have sex are worthy of killing off, while those who remain virgins or are truly in love are allowed to escape. At least in teen movies.
When we venture into other films, we often see a blending of erotic thrills with horrific ones.
But I’ll be back with a few more thoughts on that later.