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Only counting people who were Hogwarts students at he same time as Harry, I can only think of four sets of students with at least one sibling: the Weasleys, Colin and Dennis Creevey, Padma and Parvati Patil, and Astoria and Daphne Greengrass. Every other student never mentions a sibling, nor does a sibling ever appear. Presumably, any siblings of kids near Harry's age would have been close enough in age to attend Hogwarts at the same time as him.

There are slightly more adults with brothers and sisters (i.e. Bellatrix, Narcissa, and Andromeda Black, Sirius and Regulus, and the Dumbledores), but those could more plausibly go unmentioned.

According to the UK's Office for National Statistics, in 1996 only about 42% of (Muggle) families had one child, whereas a large majority seems to have one in the wizarding world.

I would prefer answers based on the books, JKR quotes, or Pottermore to hopefully explain the seemingly large number of only children.

  • 12
    Weasleys kinda average out everyone else :) – DVK-on-Ahch-To Aug 8 '14 at 0:05
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    Also, your counts are definitely wrong, I can recollect at least 2 more sibling pairs of Harry's age immediately: Greengrasses and Delacour; as well as adult pairs (Carrows, Lestranges, Gaunts). There may be more on Black Family Tree, but I'm too lazy to check – DVK-on-Ahch-To Aug 8 '14 at 0:06
  • @DVK I didn't think of Astoria Greengrass because she was never mentioned by name in the books, but I suppose she counts. – NinjaBearMonkey Aug 8 '14 at 0:10
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    Just because we never read about their siblings doesn't mean they don't exist. – Kevin Aug 8 '14 at 0:13
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    You could use this argument in almost any book - how would it read if every character is introduced along with a count of siblings - "Harry looked and saw Cho [2 sisters and 1 brother]". It just isn't relevant information for most of the characters in the books – HorusKol Aug 8 '14 at 3:44
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  1. Siblings are more common than you seem to realize. In addition to 3 you listed for HP's generation, there are at least 2 more real ones (Greengrass sisters and Delacour sisters); one "unmaterialized" one (Hermione's sister was planned by JKR) and a bunch of adults (Carrows, Lestranges, Gaunts). We also see that next generation has siblings (Harry's 3 kids and Ron's 2 and Angelina's 2)

  2. Most kids we see in the books aren't conclusively known to have or not have a sibling - they simply do not have enough "screen" presence (whatever you call that in a book?). With rare exception of ones whose family life we know very well - Luna, or Neville - we don't know for sure that they don't have siblings. Case in point, Greengrasses, who only came out to be known by JKR's post-book statements about her sister marrying Draco Malfoy.

  3. Most kids we see are the children of a reasonably young generation (Harry's parents had him at age 20). As such, many of them are likely to have been first kids and may have had younger siblings after the book. Remember that wizards seem to live longer than Muggles and this likely start families later and can afford to have kids with bigger intervals... except for Weasleys who are secretly Irish Catholic <MontyPython/> or Uncompliant Polish <Enderverse/>

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    I didn't count the Delacours because they are French, and there is no way of knowing the number of siblings at Beaxbatons. This was for Hogwarts specifically. – NinjaBearMonkey Aug 8 '14 at 0:19
  • 2 additional points - Rowling doesn't even bother naming a lot of students, like the first years buying the twins' "lunchboxes" - and at least some muggle-born are the only ones in the family to have magical ability, so some "solo" kids at Hogwarts may actually have siblings. – HorusKol Aug 8 '14 at 3:47
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    all in all - this leads us to simple 'reporting bias' – HorusKol Aug 8 '14 at 3:47
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I have to agree with you on the question and that is why I up-voted. It is never explained in the book, but I will get you some canon quotes to support my thoughts(later).

Less likely theory:

It is never said that these people don't have siblings.

Reasoning:

Imagine if you are the sibling of Neville. Well you probably died when Neville's mother(if you were in the womb) got tortured.

If you are Draco's or a deatheater's child's sibling and you were a squib, then odds are you were killed.

If you were Luna's sister or someone's else's then you might of died when they were little.

Or you might be the grownup sibling or you might be a squib.

Either way you aren't that important to the plot and siblings don't mention their family every five seconds. Also, if you don't have that much screen time, then you aren't going have much time to mention your family.

More likely theory:

The wizarding population isn't that large, as people are dying and since a lot of people don't want to marry muggles they either die out or inbreed. So the decline in birth rate might be what is happening. I put more stock in this claim.

In the book, it says that wizards are less populated as they used to be and they are inbreeding. This means a decline in birthrate(partially the inbreeding). If the wizards are having trouble having kids then less people will be going to school. I mean it seems logical less kids that are born, means people are less likely to have multiple births. And therefore less people are seen, because less kids are alive.

Reasoning

"Most wizards these days are half-blood anyway. If we hadn't married Muggles we'd've died out." —Ron Weasley

It might be because of declining wizard blood makes less wizards, so less students with siblings go to school.

Inbreeding.

Wizards that are inbreeding are going to have less kids.

"The pure-blood families are all interrelated. If you’re only going to let your sons and daughters marry pure-bloods your choice is very limited; there are hardly any of us left." —Sirius Black

But I will get some more canon quotes.If I can.

Offspring of biologically related persons are subject to the possible impact of inbreeding, such as congenital birth defects.

So from here we can see that wizards that are inbreeding are going to have less kids.

Also, inbreeding can cause these problems.

Reduced fertility both in litter size and sperm viability

Increased genetic disorders

Fluctuating facial asymmetry

Lower birth rate

Higher infant mortality

Slower growth rate

Smaller adult size

Loss of immune system function

Read the bolded stuff that alone explains why a lot of people may not have a lot of kids.

Wealth

A lot of the wizarding families are rich. A lot of rich people don't have a lot of rich kids.

Or they have reasonable wealth, not all of them has to be Malfoy rich. England is not a extremely poor country.

ONE of the paradoxes of human biology is that the rich world has fewer children than the poor world.

So the reason for lower birth rate can be because Incest(inbreeding) and Wealth.

  • A contributing factor for the pure-bloods may be the tendency of people who are self-important to be self-indulgent as well, leading to fewer children (and often just one or even none at all). Ask Charles Darwin how that's going to work out. – EvilSnack Jul 24 '16 at 21:21

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