Clare

The Statute of Secrecy prevents the population of wizards from growing too big. 

The birth rate of wizards significantly decreased in the late 1970s during Voldemort’s first rise in power, leading to fewer students in the ‘90s when Harry was in school.

The number of Hogwarts students can be estimated by multiplying the total boys in Harry’s year and House (5) by two to account for boys and girls, by seven to include all years, and by four to include all houses. 5x2x7x4=280 students. Even rounded to 300, this seems astonishingly low. 

The wizard population in Britain can be extrapolated by the approximate Hogwarts population using the statistic that 13-16% of human populations are between the ages of 11 and 17, approximating ~1800-2500 wizards in all of Britain (again, this seems low, but it can be explained by the next theory):

A lot of wizards are homeschooled, so the Hogwarts student body isn’t indicative of the entire wizarding population.

While I acknowledge the very real phenomenon of lower birth rates during times of global hardship (war, disease, etc.), and it’s likely that a lot of wizards in the UK don’t go to Hogwarts, I find that most theories don’t take into account two important factors: Harry’s unreliable narration, and a tiny peek into trends of population genetics.

🌱 Harry as an unreliable narrator:

The size of Hogwarts castle, along with descriptions of the student body, seems to be at odds with the few students we actually meet in the narrative. However, some key moments in Harry’s narration lead me to believe that we’re not getting the full picture of how many students there actually are:

1. Harry doesn’t know everyone in his year. We’re introduced to new students of all ages in The Order of the Phoenix via Dumbledore’s Army. But Harry’s introductions to students in his own year are what I find interesting— particularly the Ravenclaw boys and Susan Bones.

When students are entering the Hog’s Head during the first D.A. gathering, Harry notes the presence of “three Ravenclaw boys he was pretty sure were called Anthony Goldstein, Michael Corner, and Terry Boot” (OoTP, p 338). At the least, we know that Terry Boot and Anthony Goldstein are in Harry’s year (they’re sorted in The Philosopher’s Stone). Presumably they’ve shared classes (the Gryffindors share classes with the other two Houses), yet Harry isn’t fully acquainted with them. This is also the case for Blaise Zabini, a Slytherin in Harry’s year with whom he’s had classes but somehow does not meet until The Half-Blood Prince. 

Susan Bones is also in Harry’s year (she’s also sorted in PS). We know that Harry has had class with the Hufflepuffs (we meet Justin Finch-Fletchley Herbology in The Chamber of Secrets), yet when she enters the Hog’s Head she’s only described as “a Hufflepuff girl with a long plait whose name Harry did not know” (OoTP, p 338).

Two conclusions could be drawn from the fact that Harry doesn’t know his own peers: 1. He’s exceptionally unobservant, and 2. The student body at Hogwarts is large enough that Harry hasn’t yet encountered some of his classmates. 

I choose to believe a bit of both. We know that Harry can be observant, particularly in tense or dangerous situations (recalling the location of Ravenclaw’s diadem in The Deathly Hallows demonstrates some pretty high detail retention). On the other hand, he’s not always the most socially astute, which could allow for the argument that he simply hasn’t bothered to learn the names of some of his classmates.

Personally, I find it unlikely that after more than four years of study with a presumed class size of 20 (2 houses, 10 from each), Harry wouldn’t know the names of these students. I think it’s more likely that the classes are much larger, and that Harry’s year of Gryffindor boys just happens to consist of five students. 

2. We never get a clear idea of the Hogwarts student body. Harry’s narration often describes “hundreds” of students at Hogwarts: filling the Great Hall, the stands at school Quidditch matches, and the stadium built for the Triwizard Tournament. “Hundreds” doesn’t give us much to work with (Harry also mentions that the Christmas feast includes “hundreds of fat turkeys” in PS), but I don’t think it makes sense to only use the number of boys in Harry’s dorm for reference. The Sorting Hat divides students based on their innermost traits and talents, which shouldn’t mean that it fills a quota every year for each house. It’s likely that there’s a huge variety in the number of students per dorm, if the Sorting is legitimate.

The sheer size of the castle, the fact that there are four Quidditch teams with reasonably good players, and the fact that so many students don’t know each other are all reasons to believe that Hogwarts’ student body is much larger than 300 students across seven years. 

Harry’s a kid who often has a lot on his mind; it’s unlikely that he’s introducing us to enough students for us to obtain an accurate gauge of class size from the text.

🧬 The heritability and population genetics of being Magical:

This won’t get too dry, I hope promise. Regardless of whether you accept the above “large student body at Hogwarts” theory or prefer the “lots of children are homeschooled” theory, the likelihood of a small wizarding population in the UK (and, by extent, the world) is slim. We know a few things to back this up:

It’s not that uncommon for a wizard to be born to two Muggles (the Muggle-born Registration Commission is proof of this).

The occurrence of Squibs (non-magical people born to two magical parents) seems to be exceedingly rare (Ron mentions this in CoS).

Anyone who is a wizard (pureblood, half-blood, Muggle-born) can reasonably expect their child to possess magical abilities, regardless of the other parent’s abilities.

I want to look at magical ability through the context of what we know about genetics and heritable traits. Regardless of blood status, we see only two varieties of magic: Magical and Not Magical. Basically, you’re either a wizard or you’re not. Thus, we can use a Mendelian genetics model as a basis for comparison.

When it comes to genes that present two distinct variants of a heritable trait, there is often a “dominant” allele (a version of the gene) that will present itself even when combined with the “recessive” allele. A common example is eye color: if you receive one copy of the dominant brown eye allele, then you’ll have brown eyes, regardless of the allele you’ve received from the other parent. This can result in multiple different genotypes (genetic makeups) for just two phenotypes (visible traits): Magical and Not Magical. Statistically, that should mean that the half-blood wizards should produce children with the following ratios (sorry to resurrect biology class, but genetics is my job and I love it):

Let’s use M to indicate a dominant Magical copy of the “magic gene,” and m to denominate Not Magical. Thus, both genotypes MM and Mm would encode the Magical trait, while mm would be Not Magical.

As shown above, a cross between two half-blood wizards (Mm and Mm) would statistically produce one non-magical child (or Squib) per every 3 magical children.

However, the three facts listed above don’t support this model of magical inheritance. Half-blood wizards expect their children to be magical with the same confidence as pure-bloods; we don’t hear about enough Squibs to support this 3:1 ratio. And two Muggles, presumably with no magical genes between them, can produce a fully-functional wizard. “Magical” seems to work like some kind of all-powerful gene: it can be fully expressed down an entire bloodline with just one copy introduced by one parent.

This breaks all of the models we commonly use for population genetics, which assume that gene variants have to come from somewhere (hence the term “gene pool”), and can’t be created out of thin air other than by random mutation (i.e. Hardy-Weinberg). Instead, new magical bloodlines are constantly being created through Muggle-born wizards, and very few are terminated with the birth of Squibs, indicating that the population of magicfolk will only grow.

Perhaps wizards did see a dip in population during the wars. But they’re still human. And humans procreate; as a species, it’s what we do. Based on the above logic, by far the most likely scenario is that the population of wizards has only continued increasing, rather than declining or even remaining steady.

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🌱 Lastly, I’ll mention a few more points that support the idea of a large UK and global wizard population, mostly regarding the infrastructure we see:

The UK Ministry of Magic is a huge governing body with hundreds of employees.

The British and Irish Quidditch League alone comprises thirteen teams of professional athletes and reserve players, the funding for which presumably comes from in-person match attendance.

The Quidditch World Cup stadium was built to hold one hundred-thousand seats, yet Arthur is still considered lucky to have obtained tickets.

TLDR: I think there are a lot of wizards out there! And Hogwarts is probably much bigger than we think.

This series is full of plot holes (as are most works of this magnitude; that’s ok), but I think it’s worth it to make meaning from the text we’re given. I’m not expecting to change anyone’s headcanon or assume that fics should be written in one particular way, but I found this fun to think about. I’ve no doubt left out a lot of points (some for brevity’s sake), which I’m happy to discuss! Let me know what you think!

— GreenhouseThree 🌿