Crooked Teeth in Japan — Is the "Inbreeding" Theory True?

"Japanese people have crooked teeth" — it's a common perception in Korea, often followed by "because of inbreeding." Sounds plausible, but is it actually true?

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Are Japanese teeth actually worse?

Numbers first.

Factor	Korea	Japan
Malocclusion rate	~60-70%	~70-80%
Orthodontic treatment rate (20-30s)	~35-40%	~20-25%
Cost	4-8M won	¥600K-1.2M (~7-14M won)
Insurance coverage	Partial	Almost none

Japan's malocclusion rate is higher — that's true. But the gap isn't dramatic. What really amplifies the visible difference is treatment rates. Koreans get orthodontics far more often, so aligned teeth are more common among adults.

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The "Inbreeding" Theory — Where It Comes From

The claim goes roughly like this:

1. Japan is an island nation with a narrow gene pool → more inbreeding → more genetic anomalies
2. The Japanese imperial family and aristocracy did practice consanguineous marriages historically
3. By analogy with the Habsburg jaw deformity in Europe

This theory spread in Korea partly because it merged with anti-Japanese sentiment — "Japanese people = genetically inferior" with "teeth" as evidence.

Fact Check

Does inbreeding directly cause malocclusion?

Genetically, inbreeding increases the probability of recessive genetic conditions. Jaw-tooth size mismatch (a major cause of malocclusion) does have genetic factors. However:

• Malocclusion is polygenic. It doesn't link to inbreeding like single-gene disorders

• Japanese consanguineous marriages were limited to royalty/aristocrats. Commoners were unaffected

• Modern genetics shows no evidence of unusually low genetic diversity in Japanese people

• Other island nations (UK, Iceland) don't show elevated malocclusion

Conclusion: the direct inbreeding → malocclusion causal chain has no academic support.

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Actual Causes

1. Jomon-Yayoi Admixture (Most Accepted Theory)

• Jomon people: Original inhabitants of Japan (~16,000 years ago). Small jaws, small teeth

• Yayoi people: Migrants from Korean peninsula/China (~2,300 years ago). Large jaws, large teeth

Modern Japanese are a mix. The problem occurs when someone inherits Jomon-size jaws but Yayoi-size teeth — big teeth crammed into a small jaw.

Koreans have less Jomon genetic contribution, so the jaw-tooth mismatch is less severe.

2. Dietary Changes

Global phenomenon, but pronounced in Japan:

• Traditional Japanese food is soft (tofu, fish, udon, porridge)

• Modern processed foods are even softer

• Less chewing → underdeveloped jaw → insufficient space for teeth

Korea's relatively tough foods (kimchi, dried fish, etc.) may contribute to better jaw development.

3. Orthodontic Attitudes (Biggest Visible Factor)

Factor	Korea	Japan
Perception	"Obviously you should do it"	"Nice to have but not essential"
Cost burden	Partially insured, relatively affordable	Out of pocket, expensive
Social pressure	Straight teeth = basic grooming	Yaeba = can be charming

In Korea, orthodontics is almost a rite of passage. In Japan, it's a choice.

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Yaeba Culture — Crooked Teeth as Attractive?

The hardest part for Koreans to understand.

In Japan, yaeba (protruding canines) is traditionally seen as cute. Idols, voice actors, and celebrities with yaeba are common. In 2011, dental clinics offered artificial yaeba implants.

But this is changing fast. K-pop's influence, social media comparisons, and invisible aligner options are pushing orthodontic awareness up among younger Japanese.

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How Japanese People View This Topic

• Self-deprecating awareness: "Japan is a country of bad teeth" is a common self-critical meme

• On the inbreeding theory: not taken seriously in Japanese academic or public discourse

• Japanese Dental Association position: malocclusion is caused by genetic jaw-tooth mismatch + environmental factors (diet, mouth breathing); no academic papers support an inbreeding link