Why did anybody seriously believe that Hagrid was the Heir of Slytherin?

Question

For 50 years Hagrid took the rap for the opening of the Chamber of Secrets. Everybody took Tom Riddle's word for it that Hagrid was the Heir of Slytherin. Yet everybody seemed to skim over one vital fact - Hagrid wasn't in Slytherin.

It has been confirmed by Rowling herself that Hagrid was in Gryffindor. Yet all the teachers (bar Dumbledore) and the Ministry of Magic seemed to be perfectly content that Hagrid was the Heir of Slytherin. The Heir was supposed to be a direct descendant of Salazar Slytherin himself! Why was the prospect of a non-Slytherin Heir taken seriously?

Riddle himself expressed surprise that his story was believed.

“Hagrid’s my friend,” said Harry, his voice now shaking. “And you framed him, didn’t you? I thought you made a mistake, but -”
Riddle laughed his high laugh again.
“It was my word against Hagrid’s, Harry. Well, you can imagine how it looked to old Armando Dippet. On the one hand, Tom Riddle, poor but brilliant, parentless but so brave, school prefect, model student...on the other hand, big, blundering Hagrid, in trouble every other week, trying to raise werewolf cubs under his bed, sneaking off to the Forbidden Forest to wrestle trolls...but I admit, even I was surprised how well the plan worked. I thought someone must realise that Hagrid couldn’t possibly be the Heir of Slytherin. It had taken me five whole years to find out everything I could about the Chamber of Secrets and discover the secret entrance...as though Hagrid had the brains, or the power!"
_{(Chamber of Secrets, Chapter 17, The Heir of Slytherin).}

Hagrid may be have had a love of large monsters; I understand why that would make the teachers suspicious of him when they heard Riddle's story. But the fact still remains that for Hagrid to be the Heir a Gryffindor would've had to have succeeded in finding and opening the Chamber where generations of Slytherins had failed. Wasn't that implausible? Dippet seemingly took the myth of the Chamber of Secrets seriously; Dumbledore certainly did. Yet they expelled someone who was in the wrong house to have committed the crime.

The same mistake is made 50 years later when Harry (a Gryffindor) is suspected of being the Heir.

Why was the notion that the Heir of Slytherin might be a student from another house taken remotely seriously?

Answer 1

Tom Riddle was persuasive enough to convince them Hagrid was behind the creature attacks, and it was a solid enough explanation that they didn't question its inconsistencies.

Tom Riddle was very well-behaved in school, and did nothing to raise anyone's suspicion of him. There was no reason why they would doubt him, despite him being the one who actually did open the Chamber of Secrets. Remember, he didn't blame Hagrid for the creature attack because he was afraid that he would get caught. Since he was such a model student, no one was going to suspect him, even if they didn't have Hagrid to blame.

The reason Tom blamed the attack on Hagrid is because they were going to close Hogwarts for the students' safety. Since Hogwarts was the only place that Tom felt somewhat at home, he didn't want it to close. He found someone to pin everything on, and once they decided Hagrid was responsible, they wouldn't close Hogwarts.

Hagrid was constantly trying to keep dangerous creatures in Hogwarts, so that would make him the prime suspect for a creature-related crime. He didn't do that particular one, but doing things with dangerous creatures that he shouldn't be is something Hagrid is certainly known for. There was no one else at Hogwarts who seemed as likely as he would to unleash a deadly creature in the school.

With such a good suspect, being accused of the crime by someone that they saw as a model student, it's easy to see how they might ignore small discrepancies in their theory. If a well-behaved prefect says that the boy who kept bringing dangerous creatures into Hogwarts brought another dangerous creature into Hogwarts, there would be no reason to doubt it.

Tom also knew Hagrid was keeping Aragog in Hogwarts at the time, and in addition to saying that Hagrid was the culprit, said Aragog was the creature involved. Acromantulas are more than capable of killing a person.

So Tom had led them to both a boy who kept bringing creatures to Hogwarts, and a creature capable of killing someone. While the death wasn't because of Hagrid, Aragog being in the Hogwarts dungeons actually was. It's a reasonable, although wrong, presumption that Hagrid was responsible, and Aragog was the creature that caused the death.

The creature in the Chamber of Secrets was actually a basilisk, but without knowing this, after being informed of an Acromantula in Hogwarts, it's reasonable to presume that it's probably the creature they're looking for. Although Hogwarts could be a weird and even dangerous place, students bringing deadly creatures wasn't exactly a common occurrence.

They were given both the suspect and the creature by someone who seems to be a credible source. After already having evidence of all this, they would have no reason to think they need to look for another person or another creature. They might not have known all the details about how Hagrid was able to do it, but they'd be pretty sure that it was him.

With all the evidence stacked against him, no one would have been too concerned about whether Hagrid was the Heir of Slytherin or not. Even if they noticed that Hagrid was a Gryffindor and therefore should not be the Heir of Slytherin, they might dismiss it easily.

It would be more plausible that they were simply missing something on how Hagrid was able to get into the Chamber of Secrets, rather than they were wrong to suspect him. Hagrid might have found a way to open the Chamber without being the Heir of Slytherin. He could have broken into it somehow. They might think that he had sufficiently strong Gryffindor traits that despite being the Heir, he ended up in a different House.

More likely, they won't have even thought or cared about any of this. Something like how implausible it would be for Hagrid to be the Heir of Slytherin would be easily hidden by the amount of evidence that pointed to Hagrid. It's more logical to think that he got into the Chamber of Secrets somehow, than to suddenly consider all the evidence pointing to Hagrid being the one behind the attacks invalid, including his having an Acromantula. It wouldn't make sense to question their otherwise very solid case against him.

For that matter, were they even trying to figure out who was the Heir of Slytherin? Everyone's priority would have probably been figuring out what was harming students. Once they were fairly sure that was caused by Hagrid, the Chamber wouldn't necessarily have mattered to them.

They probably weren't concerned with the opening of the Chamber by itself - they were worried about what could have been in the Chamber, and if it was causing the attacks. Any concerns they had about the Chamber of Secrets being opened was because they thought it was where the dangerous creature came from. Presuming Hagrid was the one who brought the creature that was attacking students into Hogwarts, which seemed to be true, it would have been inconsequential whether or not he opened the Chamber or was the Heir.

They wouldn't have even had reason to suspect they were wrong retrospectively because once Hagrid and Aragog were removed, the attacks on students stopped. That was because Tom didn't want to disprove the excuse he made, but it would have looked to them like they had found the person responsible.

In addition, these weren't professional detectives who were investigating the attacks on the students. The people in charge of finding who was responsible were the Ministry of Magic and the teachers at Hogwarts. Their goal would have been to find the culprit behind the attacks and remove them. No one involved would have wanted to close down Hogwarts, and the solution they were given avoided that happening.

The Ministry, even when it has the right intentions, has never been known to be the most capable institution. Likewise, the teachers at Hogwarts would want to keep their students safe, but might not be able to pick up on small details like Hagrid being unlikely to be the Heir of Slytherin. If they were talented investigators, they might have also realized that Acromantulas couldn't Petrify, and therefore Aragog couldn't be the creature involved in the attacks.

While we have the benefit of knowing the truth, with the knowledge that they had at the time, they came to a fairly reasonable conclusion based on the evidence they had, which was quite convincing. They might not have had absolutely conclusive proof that Hagrid did it, but they had more than enough information to think he did.

Basically, Tom Riddle explained it well himself. Without knowing who actually did it, Hagrid certainly looked like the most likely suspect. Tom was a respected student who said Hagrid did it, and although Tom himself thought the idea that Hagrid was the Heir of Slytherin ridiculous, the idea that Hagrid would be the one who released a creature that was responsible for the death of another student was convincing enough to cover that discrepancy or have it written off as unimportant. Tom kind of underestimated how well he had built up his case against Hagrid.

Answer 2

The authorities did not know about the Chamber of Secrets or the Heir of Slytherin

While it was well-known the second time around that the attacks were related to the Chamber of Secrets and the Heir of Slytherin, there is no evidence that the authorities in Riddle's time knew of any connection between the Chamber, the Heir, and the attacks.

Indeed, the only reason it was known the second time was that it was advertised:

Something was shining on the wall ahead. They approached slowly, squinting through the darkness. Foot-high words had been daubed on the wall between two windows, shimmering in the light cast by the flaming torches.

THE CHAMBER OF SECRETS HAS BEEN OPENED. ENEMIES OF THE HEIR, BEWARE.

When Riddle opened the Chamber while still a student in Hogwarts, he would likely not have advertised this. Advertising it would make it more likely that the plan would get thwarted and it would also greatly increase his chances of getting caught. He was clearly in Slytherin, and it was certainly possible that someone (probably Dumbledore?) could figure out his lineage. Riddle would likely not have risked this. Indeed, he shut down the entire plan just because he wanted to be able to stay at Hogwarts over the summer.

Moreover, it is evident that even the second time around people were hesitant to attribute the attacks to the Chamber of Secrets/Heir of Slytherin. We see Professor Binns still insisting that the entire existence of the Chamber and its monster was just a legend, and it doesn't seem that he was unique in this belief. And this was after a girl was killed by the monster.

Moreover, in the entire discussion between Riddle and Dippet about the situation there is no mention of the Chamber or the Heir. They merely refer to it as "the current circumstances", "all these attacks", "the recent tragedy", and "this unpleasantness":

“The thing is, Tom,” he sighed, “special arrangements might have been made for you, but in the current circumstances. ...”

“You mean all these attacks, sir?” said Riddle, and Harry’s heart leapt, and he moved closer, scared of missing anything.

“Precisely,” said the headmaster. “My dear boy, you must see how foolish it would be of me to allow you to remain at the castle when term ends. Particularly in light of the recent tragedy . . . the death of that poor little girl. ... You will be safer by far at your orphanage. As a matter of fact, the Ministry of Magic is even now talking about closing the school. We are no nearer locating the — er — source of all this unpleasantness. ...”

Riddle’s eyes had widened.

“Sir — if the person was caught — if it all stopped — ”

“What do you mean?” said Dippet with a squeak in his voice, sitting up in his chair. “Riddle, do you mean you know something about these attacks?”

“No, sir,” said Riddle quickly.

But Harry was sure it was the same sort of “no” that he himself had given Dumbledore.

Dippet sank back, looking faintly disappointed.

“You may go, Tom. ...”

When Riddle confronts Hagrid, Hagrid does not defend himself by arguing that he could not have been the Heir of Slytherin and Aragog is not the monster from the Chamber. He merely asserts that Aragog didn't do it and would never do something like this:

“It never killed no one!” said the large boy, backing against the closed door. From behind him, Harry could hear a funny rustling and clicking.

“Come on, Rubeus,” said Riddle, moving yet closer. “The dead girl’s parents will be here tomorrow. The least Hogwarts can do is make sure that the thing that killed their daughter is slaughtered. ...”

“It wasn’t him!” roared the boy, his voice echoing in the dark passage. “He wouldn’! He never!”

This makes sense if we assume that Hagrd had no idea this was connected to the Chamber/Heir.

Harry himself only found out that the Chamber of Secrets had been opened previously from Dobby:

... And now, at Hogwarts, terrible things are to happen, are perhaps happening already, and Dobby cannot let Harry Potter stay here now that history is to repeat itself, now that the Chamber of Secrets is open once more — ”

Dobby froze, horrors truck, then grabbed Harry’s water jug from his bedside table and cracked it over his own head, toppling out of sight. A second later, he crawled back onto the bed, cross-eyed, muttering, “Bad Dobby, very bad Dobby ...”

“So there is a Chamber of Secrets?” Harry whispered. “And — did you say it’s been opened before ? Tell me, Dobby!”

Dobby probably knows this because of inside information he picked up while working for the Malfoys, as we know tha the Malfoys knew about the Chamber of Secrets:

“You know I haven’t, Goyle, how many times do I have to tell you?” snapped Malfoy. “And Father won’t tell me anything about the last time the Chamber was opened either. Of course, it was fifty years ago, so it was before his time, but he knows all about it, and he says that it was all kept quiet and it’ll look suspicious if I know too much about it. But I know one thing — last time the Chamber of Secrets was opened, a Mudblood died. So I bet it’s a matter of time before one of them’s killed this time. ... I hope it’s Granger,” he said with relish.

However, there is no reason to assume that any Hogwarts/Ministry authorities were aware of this while Riddle was in school.

When Harry and Ron visit Aragog, Aragog does make it sound as though the people fifty years ago knew about the Chamber and the Heir:

“But that was years ago,” said Aragog fretfully. “Years and years ago. I remember it well. That’s why they made him leave the school. They believed that I was the monster that dwells in what they call the Chamber of Secrets. They thought that Hagrid had opened the Chamber and set me free.”

However, this could simply be Aragog's interpretation of the events, with him assuming that the people knew about the Chamber, when in reality they had simply believed that Hagrid had a generic monster that was behind the attacks.

It is possible that Dumbledore knew about the Chamber the first time around. He does explicitly acknowledge that it has been opened before:

“What does this mean, Albus?” Professor McGonagall asked urgently.

“It means,” said Dumbledore, “that the Chamber of Secrets is indeed open again.”

Madam Pomfrey clapped a hand to her mouth. Professor McGonagall stared at Dumbledore.

“But, Albus ... surely ... who?”

“The question is not who,” said Dumbledore, his eyes on Colin. “The question is, how. ...”

And from what Harry could see of Professor McGonagall’s shadowy face, she didn’t understand this any better than he did.

However, this does not necessarily mean that he knew it at the time, and even if he did it is possible that no one else believed him when they heard his fanciful ideas about a secret chamber with a legendary monster, or perhaps he never even told anyone because he didn't expect anyone to believe him. (The fact that he says that it's a question of how and not who seems to imply that he knew that Voldemort was behind the attacks, and therefore that Riddle was the culprit the first time.)

As for Riddle's statement that he was surprised that no one realized Hagrid couldn't be the Heir or have figured out how to open the Chamber, that may simply be him talking from his perspective. He knew that the Chamber of Secrets and the Heir of Slytherin were involved so he projected that onto others, without realizing that they didn't know this information. Alternatively, he was surprised that they didn't figure out that the attacks were related to the Chamber/Heir and that therefore Hagrid couldn't have been the culprit.

As for why Hagrid was arrested the second time around (once everyone knew about the Chamber/Heir), it may simply be because it was already accepted that he was guilty the first time so it was assumed that he was the perpetrator again, even though it was highly implausible. Additionally, it is likely that even Fudge didn't really believe that Hagrid was guilty but just needed to do something.

As for why people suspected Harry as being the Heir, there was some evidence in favor of such a theory. After all, he was a Parselmouth which was the trademark of Slytherin, and he was the only known one to have been at Hogwarts. Combined with the fact that he had defeated Voldemort, which may have been evidence of dark powers, and he was discovered at the scene of the crime multiple times, and one of the attacks was against someone who had been harassing him, and there weren't many other candidates, this may have helped people believe that he was the Heir.

Answer 3

I believe the question is specifically about the Heir of Slytherin being from another house.

There is no reason why they would not be.

The sorting has nothing to do with one's ancestry or bloodline. Sorting counts only one's personality. Thus, nobody is surprised much that twins Patil are sorted to different houses or Sirius Black gets to Gryffindor. All Ron's family is in Gryffindor and still he is nervous that he might be sorted differently. Because, again, your ancestry does not define your personality. Especially if the ancestor is so distant in time. The fact that families usually get sorted into the same house probably depends more on their shared views and values that are common for the family than on their blood relativeness.

Moreover, as we see with Harry, sometimes the hat sees a student fit for several houses and can go with the kid's choice. There are a lot of students who could have been sorted into other houses (like, McGonagall or Hermione), so there will be not a big stretch to assume there might be a Ravenclaw or Gryffindor who could equally be fit for Slytherin.

People were fast to believe that Harry Potter was the Heir of Slytherin. He almost believed it himself actually.

There is just no reason why anyone would consider it impossible for Hagrid to be the Heir of Slytherin only because of him being in Gryffindor.