Mind Tricks Demystified
Mind control is a critical part of the Potter series, and is a fascinating topic in and of itself. There are a number of minor issues relating to the subject, seeming inconsistencies, and unanswered questions for us theorists to chew on. This essay will attempt to tie them up. Mind control comes in two basic varieties—"external" mind invasion, such as the Imperius Curse, and psychic possession, in which the wizard's very soul is occupying and directing another person.
It's stated in the very first book—not just stated, but demonstrated—that it is possible for a person, or animal, to be possessed by the soul of a wizard. Professor Quirrell's tragic saga puts psychic and physical possession on the menu immediately. And then in the following book, we get a depiction of the possession of Ginny Weasley, this time by a different part of Voldemort's soul.
The two incidents appear to have operated differently. Quirrell, as far as we can tell, didn't lose his memories of being possessed. We don't know this for sure, since he died soon after the possession ended, but given that in the third instance of human possession in the books—that of Harry in the Atrium—the victim didn't lose his memory of the event, that could suggest that something else was going on with Ginny. He was either wiping her memories after he possessed her, or his possession of her was of a sort that forced her consciousness into dormancy while he was in control.
With Ginny, it would have been in Tom's interest to wipe her memory each time he used her. He couldn't well leave her with knowledge of everything. We know that Tom, the same summer after he created the Diary, was competent enough to implant a false memory in his uncle. We also believe that the Diary functioned as a Pensieve in addition to its more sinister aspects. Tom seems to have been highly proficient with memory tampering. I think Ginny was sadly mistaken when she tells Harry that you can't remember being possessed. What Tom did to her was an unusual case.
Voldemort was also possessing Nagini during the snake attack sequence in book five. I imagine that Voldemort's body disappeared while this was going on. The Horcrux was there, yes, and Voldemort could have instructed the snake through the connection without actually possessing her. But Harry has never perceived himself as the snake except that once, while he has repeatedly perceived himself as Voldemort. The connection is between the "master" fragment and the living Horcruxes, not between one 'crux and another.
I have a very strong suspicion that it is only possible to possess someone if your soul is already grounded by a Horcrux. Otherwise it seems that it would be pulled through the Veil with nothing to bind it to earth. We do know that the individual Horcruxes can possess the unwary, and that according to Hermione's book, any Horcrux is capable of it, so it isn't a skill that only Tom Riddle can do (although he does seem to have a particular aptitude for it). You'll notice, too, that the only person in all seven books who is stated to have possessed people is also the only person who has created a Horcrux.
Imperius and Other Dirty Tricks
Other than brief, detail-less mentions of Voldemort inhabiting animals in the forest during his exile, we don't have any other instances of actual possession in the series. There is plenty of Imperiusing and mind control, though, which appears to follow different rules from possession. Unfortunately, the Imperius Curse itself is written in an inconsistent manner. Barty Crouch Sr., Madam Rosmerta, and Pius Thicknesse (at a minimum) could be put under the curse for a long period of time, with their controller at a great distance from them at times, and no one noticed notable changes in their behavior in public (at least, until Crouch began to fight the curse and went mad from it). However, Harry had to point his wand at his Imperius victims or they would stand idly by and look blank. There is no explanation given for it.
We can make something up that almost fits. Harry, after all, is an inexperienced caster. He did do the curse well enough to control a rather mentally sharp Death Eater and a Gringotts goblin, neither of whom exactly falls into the category of "weak-minded." Still, it works, if inelegantly. It could be that someone can be Imperiused and directed to "act normally" in times when they are not receiving other orders. It may also be possible to put someone under Imperius without putting them in a brain fog, and giving them access to their memories so that they would act normally if they were not receiving instruction. If so, this would be a more advanced Imperius Curse than the one that Harry (and Crouch as Moody) cast. If this latter explanation is the correct one, it speaks rather well of Draco Malfoy's spellcasting abilities.
On this subject, it seems that Voldemort has an ability to perform a sort of wandless Imperius Curse. Before he physically possessed Quirrell, he had his mind enslaved to his will. Voldemort didn't even have a body at this point, let alone a wand. What's more, he has had this alarming ability since before he knew he was a wizard. He tells Dumbledore in the orphanage that he can make people feel what he wants them to, and make animals do things. And Mrs. Cole's comment about the dead rabbit hanging from the ceiling also looks suspect. Given that Tom definitely had the ability to do this "wandless Imperius," it's easy to believe that he put the rope around its neck, made the rabbit jump unnaturally high, and mentally attached the other end of the rope to the rafter.
(Totally irrelevant aside here, but I find it hilarious that mind control in the Potterverse is a criminal offense worthy of lifetime imprisonment, but in the Star Wars universe, it is a skill associated with the Jedi Knights and presumably used by them with the backing of the government, at least during the days when the Jedi-friendly government existed.)
I'm highly suspicious that possession, Imperius, and wandless mind control are magics related to a fourth skill that we know of: Legilimency. Voldemort's good at that too. I strongly wonder if we have a continuum here that we're supposed to pick up on: Legilimency —> Imperius/Mind control —> Psychic possession. They all involve some amount of mental, non-verbal control over a person. To progressively more severe degrees, they allow the caster access to the subject's mind. This is a bit wild, I'll grant, but this line of thought leads me to wonder if Dumbledore isn't responsible for Legilimency and Occlumency not being taught in school. Not by banning, of course, but by removing it from the curriculum because it is a precursor to mind control and psychic possession. (There's a hint that this may have changed when Snape took the DADA class; he advocated some method of resisting Dementors that Harry disagreed with, and Occlumency is a very good candidate for it. Blocking one's emotions would be useful to deceive Dementors. But even if Occlumency is now on the Hogwarts curriculum, Legilimency certainly isn't.)
Legilimency is a teachable, learnable skill, but it is rare. It would appear that if a character has learned it, the teacher was either Albus Dumbledore, Severus Snape, or Lord Voldemort, all three of whom are known to be extremely well read. Other than these three, we think that Bellatrix Lestrange had some sort of ability with Legilimency, or she wouldn't have been able to teach Draco how to protect his mind against it. She also was able to see through Snape until the Spinner's End chapter of HBP. There is a theory that Remus Lupin was also a Legilimens, based on his instant understanding of what happened between Sirius and Pettigrew just by looking into Sirius's eyes, and reinforced by his certainty that "Snape was a very good Occlumens." I'm disappointed that this issue wasn't answered in the book before his death, but it's the sort of question that Rowling might answer in interviews, so we may not have heard the last of it.
In any case, though, we know of only three Legilimens and have two more
suspected ones. This skill is highly obscure and is not taught in
There also seems to be a profound inability among Potterverse characters to cast a functional Imperius Curse, given the discrepancy in behavior of the known victims as detailed at the beginning of this section of the essay. Even people who have demonstrated no difficulty at all in casting functional Avada Kedavras or Crucios, despite how you must want your target dead or suffering in order to make them work, seem to have difficulty getting this one right. The Muggle minister "acting like a duck" from a poorly cast curse springs to mind, as does Yaxley's report of the difficulty in putting the curse on Thicknesse. It may be that good Legilimens are best suited to cast the curse, since they already know how to get into someone's mind.
As I stated prior, I think that physical possession, forcing one's soul into someone else's body, requires grounding by a Horcrux or it will cause the death of the person attempting it. It seems natural, too, that wizards of a certain mindset might find themselves dissatisfied with the Imperius Curse, especially since it can be repelled and fought over time. These individuals might decide to research more "secure" ways of overpowering other people, and stumble upon possession in their perusal of Dark Arts books. Then they'd learn that it required a Horcrux.
If I am at all on target with this theory, Dumbledore might have thought that it was best to just discourage the whole branch of magic by removing it from the official curriculum. This would then explain why no one seemed to know much about Legilimency and why even among Death Eater ranks, there were only two (three if you count Snape as a DE) who could do it. You'll notice that Albus showed no inclination whatsoever to have Harry taught Legilimency. We know him to be cautious and very protective of secrets, and it seems likely that this was a Pandora's box that he simply did not want opened. With Harry carrying part of Tom around, and known to be able to throw off the Imperius Curse almost instantly even when Voldemort has cast it, there was no telling what ill could result from Dumbledore or Snape teaching him how to read people's thoughts deliberately.