The earliest documented instance of this story was collected in 1964 from a freshman at the University of Kansas. Since that time the story has spread mightily, and it's now uncommon to encounter anyone who hasn't heard it. As for how widespread it is, by the late 1970s, a version of it was circulating in Malaysia.
It's possible the roots of legends like The Boyfriend's Death and The Hook lie in distorted memories of real life Lover's Lane murders. There were actual cases of kids who went out necking and came back in pine boxes. The residue of news stories about those events would likely remain around for a while, mutating into cautionary tales with the addition of bloody hooks and scraping sounds on the roof of the car.
Common elements of The Boyfriend's Death include the parked couple, the abandoned girl, the mysterious scratching, tapping or dripping, the daybreak rescue by parents or police, the entreaty not to look back, and the hanging or decapitated boyfriend.
Wherever and however this story is told, the injunction not to look back is a constant. Taboos like this are always broken in folklore, whether in fairy tales or in modern urban legends. When she disobeys and looks, the girl's hair turns white from shock -- another familiar legend motif and itself a reminder of what happened to Lot's wife when she ignored a similar instruction.
The girl in this story is perceived as helpless and incapable of venturing away from the safety of the car (no one ever suggests she accompany her boyfriend), whereas her male companion immediately dons the mantle of midnight adventurer and seeker of help. The stereotyped roles of frightened female and fearless male are strictly adhered to. His job is to do battle with the unknown whereas hers is to obey orders, staying with the car no matter what until rescued. Not even thumps, bumps, or scratching noises incite her to attempt to save herself.
Interestingly, the heroine in the 1998 version quoted earlier on this page does disobey the boyfriend's orders. This lone exception in the 30-plus years of this legend's history leaves the vehicle without being instructed to do so. Even then, it's not to go looking for help or to find out what happened to her companion, it's to dispose of an annoying branch which is tapping on the car, disturbing the tranquility of her long wait to be rescued.
Variations:
Without exception, the girl remains with the car while the boy strikes out unaccompanied in search of gasoline. The murder victim is always male, and the one rescued and cautioned not to look back is always female.
In American versions, the couple is often parked under a tree, and the boyfriend's corpse is left hanging upside-down from a branch, the fingernails scraping against the car. European versions typically have the madmen himself holding the boyfriend's decapitated head in one hand and tapping on the roof of the car with his fist or bloody axe.
This legend is often said to have happened in particular lovers' lanes that are well known to storytellers and their audiences.
Sometimes when the girl looks back and sees her dead boyfriend hanging there, her hair instantly turns white.
