Dursley Family
As a family
The Dursley family is a children's book caricature, and next to impossible to make any sense of. I am not a psychologist, so all of this might be incredibly off base, but they simply do not make sense, and so, I think, have to simply be taken as the fictional constructs they are.
Petunia's description is of a nervous anorexic woman; Vernon's is of blustering, short tempered, massively obese man. Looked at from the outside, it would be hard to envision a more mismatched pair, something I think was intentionally done to give the books a sort of Roald Dahl-ish feel to them. However, if you try to make something less children's literature and more something that actually makes sense, you would swear that Petunia was an abused housewife. That does not, however, seem to be the case. She is a shrew, and, if anything, Vernon is very slightly wary of her. In first meeting them, we find that he is cautious of reminding her of her sister.1 In the fifth book, look at how quickly he backs down when Petunia says that Harry will in fact remain in the house after the dementor attack.2 Neither fits the profile I would expect if he dominated her.
Sure there are counter examples to this, where he over rides her, but the relationship is either surprisingly balanced, or even more surprisingly tilted in her direction.
How abusive were they?
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[Harry][] is "small and skinny for his age".[^221005-1]
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[Harry][]'s cupboard is full of spiders
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[Harry][]'s cupboard is "dark."[^190610-4]
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"Dudley's favourite punching bag was Harry"[^190610-2]
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Vernon is not just violent towards [Harry][]:
Dudley was sniffling in the back seat; his father had hit him round the head for holding them up while he tried to pack his television, VCR, and computer in his sports bag.[^200513-1]
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What does it mean to be "given a week in the cupboard" given he sleeps there nightly anyway?[^190610-5] Certainly that the door is locked from the outside[^190610-6] at least some of the time, but not all of it.[^190610-8]
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Vernon is not the only one offering potentially severe physical harm to Harry:
Harry paid dearly for his moment of fun. As neither Dudley nor the hedge was in any way hurt, Aunt Petunia knew he hadn't really done magic, but he still had to duck as she aimed a heavy blow at his head with the soapy frying pan. Then she gave him work to do, with the promise he wouldn't eat again until he'd finished.[^200513-2]
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Incarceration like this is illegal now, and probably frowned upon even then:
Uncle Vernon was as bad as his word. The following morning, he paid a man to fit bars on Harry’s window. He himself fitted a cat-flap in the bedroom door, so that small amounts of food could be pushed inside three times a day. They let Harry out to use the bathroom morning and evening. Otherwise, he was locked in his room around the clock.[^200513-3]
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Vernon strangles Harry in the Order of the Phoenix[^200513-4]
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[Harry][] has some disturbing habits:
“You'd need more than a good sense of fun to liaise with my uncle,” said Harry darkly. “Good sense of when to duck, more like.”[^200513-5]
and then the quote:
Harry ran down the stairs two at a time, coming to an abrupt halt several steps from the bottom, as long experience had taught him to remain out of arm’s reach of his uncle whenever possible.[^200513-6]
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On the other hand "I accept there’s something strange about you, probably nothing a good beating wouldn’t have cured"[^190610-7] suggests that [Harry][]'s abuse has been emotional in nature (along with neglect of course) and not physical.
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[Dumbledore][]'s most quoted words on the subject come from the meeting in his ([Dumbledore][]'s) office just after [Sirius][] dies.
Five years ago you arrived at Hogwarts, Harry, safe and whole, as I had planned and intended. Well - not quite whole. You had suffered. I knew you would when I left you on your aunt and uncle's doorstep. I knew I was condemning you to ten dark and difficult years.[^210128-1]
This statement is ambiguous in a number of different senses. Just what did [Dumbledore][] know, considering the Dursleys had not acted yet, what does [Dumbledore][] mean by "not quite whole," "suffered," and "dark and difficult"? Each of these can be taken a number of different ways. His next statement on the subject, in book six, sheds somewhat more meaningful light:
You have never treated Harry as a son. He has known nothing but neglect and often cruelty at your hands. The best that can be said is that he has at least escaped the appalling damage you have inflicted upon the unfortunate boy sitting between you.[^210128-2]
There are also some descriptions of Vernon's behaviour, particularly in the third chapter of the Philosopher's Stone, that may or may not be Vernon talking more aggressively than he acts, and may or may not be figurative language rather than a literal description of how he treats [Harry][]. Overall there is certainly verbal abuse and neglect. By today's standards we'd consider the above list sufficient to tack on physical abuse as well though. Harry should not have needed to learn to habitually stay out of reach for example. From my reading of other people's comments on this topic,[^211025-1] I have gathered that the standards for what constituted child abuse were more lax in the 1980s (when Harry was growing up), and even more so in Britain as compared to the United States, but I have no authoritative evidence of that, just hearsay.
In short, as another (better) author puts it,
I think the Dursleys were much more physically abusive towards their nephew - if you can grab him around the throat or swing a frying pan at his head, not to mention starving him and locking him in a cupboard, I think there is a lot more going on.[^210604-1]
While Mrs. Rowling may have chosen not to show us everything, she has shown enough that a reasonable adult should be able to see the forest as well as the trees.