Population Dynamics in the Harry Potter Universe

 In Harry Potter and the Chamber of Secrets Hagrid makes an interesting comment. He says

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

This comment got me thinking about the genetics and population dynamics of the Harry Potter universe. This topic prompted a lot of questions.  

Why is the magic population so small relative to the non-magic population? 

Hagrid's comment implies that there is something inherent in magic users that puts downward pressure on their population. There are a few possible explanations:

• A genetic effect associated with the same genes that allow magic use also reduces fertility.
• Certain magic users face cultural pressure not to have children (for example, witches might face pressure not to have children, resulting in more magic using men choosing non-magical partners). 
• Hagrid could be referring to a specific event, rather than a general trend (for example, say there was a catastrophic event in the past which eliminated a critical mass of magic users).

Let's see if we can isolate one of these potential causes. 

Size of the Magical Population

The size of the magical population is hard to gauge, but it is clearly fairly small. Let's work out an estimate. We know that Hogwarts is the only school of magic in the UK, and that attendance at Hogwarts of another school of magic is (effectively) mandatory for magic users. Based on the size of the great hall, which ostensibly contains all of the students and staff at once, Hogwarts likely has about 200 students ages 11 - 18. 

According to the UK office of statistics, people ages 11-18 made up about 10% of the total population in 1998 (about 5.9m out of 58m people). We know that witches and wizards can live longer than regular people, so let's generously assume people 11-18 make up 5% of the magic population. Based on these assumptions, the total magical population is the UK in 1998 was approximately 4,000 people, or about 0.007% of the total UK population. The UK accounted for approximately 1% of the global population in 1998. This implies a global wizarding population of about 400,000. 

The quidditch world cup is said to have 100,000 attendees in Harry Potter and the Goblet of Fire. The 2002 Football World Cup had about 2,700,000 attendees (27x as many) [5]. The world population in 2002 was 6.2 billion [6], meaning 0.04% of the world population attended the Football World Cup. One could extrapolate that the global wizarding population is 100,000 / 0.0004 = 250,000,000. However, I believe the ease of transportation in the wizarding world means a much higher percentage of the world population attends the quidditich world cup, likely as high as 25%, based on a a global wizarding population of 400,000. 

This population estimate aligns nicely with the apparent scale of the magic economy: magic users in the UK are effectively a small town. The ease of transportation enabled by magic (apparition, flue powder) means that the population can be geographically distributed over a large area without losing the cultural cohesion of a small community. A variety of observations seem consistent with the magical population existing as a distributed small town (or perhaps small nation-state):

• They have their own currency.
• They have a small number of businesses (one bank, several restaurants, several specialized stores).
• They have a single government.
• Most Magic users seem to prefer (or are limited to?) magic-specific occupations. It is implied that this is a cultural preference, rather than law. 

There a some potential inconsistencies with this view. For example, In 1998 approximately 5% of the UK population was employed in Education. [2] The magical community does not seem to follow this employment demographic, since the staff at Hogwarts is only about 0.25% of the magical population. 

Regardless, I think there are metrics to this concept that deserve more thought. Are there aspects of a small-town culture that put downward pressure on population? 

Magical Ability Propagation

There's general consensus in the literature that magical ability is inherited, though it's unclear whether the trait is expressed on a single gene or multiple, or whether it is dominant or recessive. [3] [4]. This is consistent with the potential for non-magic users to have magical children (e.g., Hermione) and the ability for magic users to have non-magical children (squibs, like Mr. Filch).

Birth Rates Among Magic Users

We don't get to meet many magical families, but with the exception of the Weasley family almost all magic families have only one child. This sample set is also likely skewed toward a higher birth rate, since most of the characters are children. 

Listing all of the adults of an age to have had children:

• Weasleys - 7 Children
• Dumbledores - 3 Children
• Hagrid's Parents - 1 Child (his half brother is not a magic user and does not count)
• Potters - 1 Child
• Malfoys - 1 Child
• Lovegoods - 1 Child
• Longbottoms - 1 Child
• McGonogal - 0 Children

Unfortunately there are a lot of characters about whom we don't know enough to make a robust data set.  For example, we aren't told whether McGonogal has siblings or children. There is not enough data to make a meaningful statistical statement about magical birth rates. Anecdotally we can only observe that many adults in the Potter books are childless (or have children that are never mentioned). Only one Hogwarts teacher ever has a child (Lupin). The rest have no mention of children:

• Dumbledore
• Hagrid
• Lockhart
• McGonogal
• Slughorn
• Snape

Education and Autonomy

It is well established that increases in education and (and presumably the resulting autonomy), particularly for women, lead to overall decreases in birth rate [7]. Although the wizarding world seems to have maintained some aspects of the patriarchy of the world at large (the ministers of magic are all male, for example), women in the wizarding world clearly have education and autonomy at a level on par, or ahead of, most modern economies. The birth rate has been steadily decreasing for many years [8].  Was Hagrid making a veiled sexist comment about witches preferring to have fewer children than their muggle counterparts? 

Recommended reading: If Hermione Were The Main Character In "Harry Potter"

Further Research

There is a lot of research yet to be done. Some ideas:

• Set up population simulation to try to determine approximate rates for the wizarding world. 
• Analyze the supply chains for magic users - how do they differ from those of non-magic users? 
• Is in-breeding a problem among magic users? The preference for "wizard blood" among some users could lead to such situations. 

[1] UK Office For National Statistics

[2] UK Office For National Statistics

[3] Live Science

[4] Muggle Net

[5] Statista

[6] Census

[7] Worldbank

[8] Macrotrends