As a subset of horror's Slasher Genre, there exists a Killer Clan Genre, though not often divided as such -- primary example being Tobe Hooper's flinch-inspiring, mesmerizing low-budget Grand Guignol nightmare, The Texas Chainsaw Massacre. That template has begat every-miserable-thing from The Last House on the Left to The Human Centipede. The simple formula: a first set of characters (friends, family, combo) wanders into questionable territory, thereby encountering (often through vehicular breakdown) a second set of Very Bad People. Hijinks ensue.
In Wes Craven's original The Hills Have Eyes we have a family on vacation, camper in tow, taking a detour through the desert to visit a silver mine. While driving, the unfolded route map literally flies into dad's face. He steers the station wagon into a stand of sagebrush, breaking the rear axle. Night falls, a band of cannibals descends from the surrounding crags. Despite the National Lampoon-esque setup, Craven conjures a harsh, effective sense of unease. The targeted characters may not be fully drawn but are at least affable; even the most annoying of them (spoiler alert: the grandmother) is capable of evoking sympathy. Whereas Craven's previous Last House was a dismal exercise in viewer endurance, Hills provides touches of genuine horror; if not a classic, at least a taut exercise.
When a film aiming for big scares wants to go all the way, it often purports to be "Based on a True Story." It's long been said The Texas Chainsaw Massacre was inspired by serial killer Ed Gein; given that Gein was one person rather than a whole family and never used a chainsaw or a leather mask and was from Wisconsin, this might be a stretch. After all, TCM was originally released in 1974 -- just enough time for a low-budget, subversive cinematic echo of the upsetting evils of the Manson Family, pinging the dark undercurrent of the American Unconsciousness. In the case of The Hills Have Eyes, Craven looked to Scottish folklore for inspiration: the grisly cannibalism of Sawney Bean. What, did somebody already have an option on The Alferd Packer Story?