The short answer:  
The idea that Victorian women hid their teeth because of sugar‑related dental decay is partly true historically, but the modern behaviour (where it exists) comes from cultural norms about modesty, femininity, and self‑presentation, not DNA.

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🧭 What’s actually going on
The behaviour you’re describing is best understood through three cultural mechanisms, none of which involve inherited reflexes.

- Victorian modesty norms — Women were expected to appear demure, restrained, and not display large, open laughter. Covering the mouth signalled “ladylike” behaviour.
- East Asian politeness norms — In Japan, Korea, and China, covering the mouth while laughing is still common, but it’s tied to etiquette and humility, not dental decay.
- Modern beauty standards — Some women cover their mouths because they feel self‑conscious about teeth, braces, or the “uncontrolled” look of laughter.

These are learned behaviours, reinforced socially, not biologically.

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🧬 Why it cannot be genetic
For a behaviour to become a reflex encoded in DNA, it must:
- provide a survival advantage,
- be selected for over many generations,
- show consistent expression across cultures.

Covering the mouth while laughing:
- does not affect survival or reproduction,
- is not universal among women,
- varies dramatically by culture, class, and era.

So it cannot be a genetically fixed reflex.

If it were genetic, you’d expect women everywhere — including those in societies with no Victorian influence — to do it automatically. They don’t.

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🕰️ Where the Victorian dental idea fits
Victorians did consume huge amounts of sugar, and dental care was primitive. Many people — men and women — had poor teeth.  
But the behavioural inheritance from that era is social, not biological:

- Women were judged more harshly for visible imperfections.
- “Modesty” was a feminine ideal.
- Open laughter was considered vulgar.

So the behaviour was reinforced socially, not genetically.

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🌏 Why the behaviour persists in some cultures today
In places where modesty norms remain strong, the gesture survives as a social signal, not a dental one.

Examples:
- Japan: covering the mouth while laughing is polite.
- Thailand: it signals humility.
- Some Middle Eastern cultures: it signals decorum.

In Western countries, the behaviour is far less common and mostly linked to self‑consciousness, not tradition.

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🔍 The deeper insight
Behaviours like this show how cultural norms can feel instinctive even though they’re learned. When a gesture is repeated across generations, people internalize it so deeply that it feels automatic — but it’s still cultural conditioning.

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A useful next step
Would you like to explore how cultural behaviours become internalized across generations or why some gendered behaviours persist even after the original cause disappears?