When Joanne Rowling first published "Harry Potter and the Philosopher's Stone" ("Sorcerer's Stone" in the US) nearly 20 years ago, she created a global cultural phenomenon. The brand is currently estimated to be worth a casual $25 billion dollars and with the "Fantastic Beasts and Where to Find Them" franchise only just getting off the ground, that figure is set to rise. The books have been translated into more than 68 languages in addition to English.
Many of us grew up alongside Harry, Ron and Hermione. Cheering for Harry when he caught his first Snitch; tears streaming down our faces during scenes where beloved characters met their ends (Dumbledore's death scene as portrayed in the film gives me goosebumps just thinking about it); staying up all night for the midnight release and then refusing to sleep until the last word was read.
It should come as no surprise then that something so prolific has had an impact on the way we socialise and create our identities. There is the famous study that shows Harry Potter fans are more accepting, tolerant and have greater empathy than other groups.
Literature with complex, developed themes and characters appears to let readers occupy or adopt perspectives they might otherwise not consider; and it seems that Rowling might get at the beautiful, sobering mess of life in a way that could have a meaningful impact on our children's collective character. — Bret Stetka, Scientific American, 2014.
So, to the Houses.
Gryffindor, Ravenclaw, Hufflepuff and Slytherin. Four words that are significant in the life of any Potter fan. Most people who have almost no knowledge of the Harry Potter world have some sense of the core characteristics of each house.
Your house speaks directly to your personality. On the surface, Gryffindors are seen as brave, if not sometimes a bit reckless in their courage. Ravenclaws are smart, high-functioning and snobby. Slytherins are cunning and ambitious. Hufflepuffs are loyal and caring.
What this presents to the more focused eye is a goldmine of psychological information. Pottermore, seen to be the definitive method of determining what house you belong in, asks a series of incredibly detailed questions that have the underlying nuance that speaks to much greater behavioural theories (see example below).
Every person who wants to be Sorted needs to go through this process of question and answer. Considering the success of both Harry Potter and Pottermore, plus the determination of many to be Sorted, there must be an incredibly large data set sitting on the Pottermore servers. Data that can tell us interesting things about the way people identify themselves.
A study done in 2015 linked key psychological traits with traits most commonly associated with Hogwarts houses. Gryffindor and their bravery translates to extroversion and openness. Hufflepuffs need social inclusion, justice and agreeableness. Ravenclaws long for higher-order thinking. Slytherins, the home of dark witches and wizards anon, was linked with the Dark Triad: narcissism, Machiavellianism and psychopathy.
"In the case of Harry Potter, Rowling's organisation of students into school communities or ''houses'' with particular traits may have influenced the self views of millions of readers and provided them with fictional communities with which they identify. It seems feasible that readers would identify more with those characters who share the traits they actually possess." — L.C. Crysel et al, Personality and Individual Differences, 83 (2015)
Interestingly, Rowling discusses this identification in the books, albeit indirectly. These calming words from Albus Dumbledore when Harry feels he should have been placed in Slytherin over Gryffindor in "The Chamber of Secrets" makes the exact case presented by the study: "It is our choices, Harry, that show what we truly are, far more than our abilities."
The findings of the 2015 study are not a value judgement but rather a qualitative psychological assessment. Also, it speaks to a tendency not necessarily a universal truth. All Slytherins don't feature the Dark Triad, but more than the average do.
The study does note that the Gryffindor result may be flawed owing to metrics used not necessarily being in line with the idea of bravery. However, those who self-identified as Gryffindor showed a greater level of extroversion.
In another study, Reddit user N1ffler did data analysis on a case study of the results of 10,000 unique Sortings on Pottermore; working out what answers lead to which houses. What the data shows is a tendency of certain people to select certain answers with a 99 percent accuracy in determining final house outcome. Considering that Pottermore was created by Rowling, and therefore is based on the original Harry Potter knowledge base, using Pottermore as a metric for house placement seems logical. The fact that the study concluded that people, for the most part, were put into the houses they personally identified with supports this.
Essentially: the Hogwarts houses act as an easy psychological rubric that most people can connect with. Where terms like Dark Triad, need for cognition and other jargon might seem otherworldly to the average person, Rowling's collective works give us characters, qualities and ideas to use as a framework in the solidification of our own identities.
Those who are placed into traditionally "bad" houses (Slytherin or Hufflepuff) have to have confronted the reality of their house's reputation and still insisted that it was where they belong. For Ravenclaws and Gryffindors, the opposite applies: dealing with the more complicated, often troubled nature of characters shown stereotypically in a positive light only.
"Rowlings' grouping of her characters into distinct houses appears to correspond to established psychological constructs, even though Rowling lacks any formal training in personality psychology (to the best of our knowledge). In addition, participants sought identification with those houses that reflected their self-perceived personality traits. The present study suggests that people can or do learn something about personality traits and fulfil identification needs through reading popular fiction. It may be that assimilating with Rowling's groups even change the way people view themselves. Thus, fans that feel transported by the Harry Potter book series may be experiencing something more like reality than mere fantasy." — L.C. Crysel et al, Personality and Individual Differences, 83 (2015)