On Japan’s Shinkansen trains, pork buns split reality in two

Each day, Japan’s high-speed Shinkansen rail network carries roughly half a million passengers between Osaka and Tokyo. Passengers begin their journey by boarding the train and, after finding their assigned seat, settling in. But, once the doors on the express trains close, there’s no stepping off again until they have reached their final stop – whether that’s two hours away or five.

Within this mobile confinement, ‘pork bun terrorism’ periodically erupts. The ‘attack’ begins innocently: a passenger purchases a steaming-hot pork bun at Shin-Osaka station, for example, anticipating the modest pleasure of consuming it during the journey. Yet, once the package is opened and the passenger begins to eat, the smell of steamed dough, onion and hot pork fat spreads through the surrounding rows. A swift – and sometimes vociferous – rebuke from a nearby passenger may follow. Sometimes, the exchange may escalate into a heated altercation.

The offending article. Courtesy Daremoshiranai/Flickr

The controversy first drew widespread attention online in 2018 and has resurfaced periodically ever since. Despite being asked to adjudicate on the matter, the railway companies that operate Japan’s Shinkansen trains have largely avoided a blanket ban on strong-smelling food, relying instead on passengers’ discretion and consideration for others. But the issue keeps flaring up.

The question of whether pork buns should be eaten aboard Shinkansen trains has sharply polarised opinion into two seemingly irreconcilable camps. On one side are the passengers who feel entitled to eat what they like on board (especially when it involves food they see as harmless), and on the other side are those who consider the unwanted smell of pork buns to be an inescapable imposition. What makes the dispute unusually combustible is that both sides regard their own experience as self-evident. Each interprets the other’s reaction as rude or unreasonable.

I find myself siding with those who oppose eating pork buns on the Shinkansen. The smell causes me intense, visceral discomfort. I have long struggled to understand how anyone could introduce such a pervasive odour into a confined space where others cannot escape it.

He regarded the complaint as an attempt by a stranger to impose a personal standard on him

By contrast, those who defend the eating of pork buns appear entirely bewildered by this response. They argue that they are merely enjoying a delicious local delicacy sold at the station and cannot understand why they are met with what they regard as misplaced anger, from critics who seem intolerant and hypersensitive.

In 2026, another case of ‘pork-bun terrorism’ made these positions particularly vivid. A Japanese entrepreneur posted online that a fellow passenger aboard the Shinkansen had reprimanded him for eating a pork bun – one of the much-loved 551 Horai pork buns sold at Shin-Osaka Station. He regarded the complaint as an attempt by a stranger to impose a personal standard on him: eating was permitted onboard, he argued, and the buns were sold hot at Shin-Osaka Station. To him, eating a pork bun was an ordinary pleasure of travel. Debate erupted once again.

Although Japan is widely known for its exacting norms of public etiquette, the pork bun problem remains an anomaly, resisting resolution through conventional appeals to manners. Why, on this particular issue, are we so incapable of understanding one another?

Research suggests that the answer has less to do with tolerance than with the fact that each side is, quite literally, experiencing a different reality.

As a neurologist who studies the brain and sensory perception, I have arrived at a specific hypothesis. A profound misconception lies at the root of this conflict. The misconception is this: because we occupy the same physical space and are exposed to the same odours, we assume that the person next to us perceives them exactly as we do. More generally, we mistake shared space for shared perception. Viewed through this lens, the structure of the pork bun conflict becomes clearer.

If we define manners as the voluntary modification of one’s behaviour out of consideration for others, then the premise of broadly shared perception often serves as a practical foundation for etiquette. When we assume that our internal experience roughly aligns with that of others, we can more readily imagine their feelings and adjust our conduct accordingly. For instance, when we wait our turn in a crowded queue, we assume that others, too, would be inconvenienced by queue-jumping. When we refrain from littering, we assume others would find a litter-strewn path unpleasant. This presumed alignment allows us to act considerately without being asked.

An intractable problem arises when what appears to be a shared experience has unknowingly fractured beneath the surface

However, we can encounter situations in daily life where this premise does not hold. Consider when someone offers their train seat to an elderly person or a pregnant woman. The person offering up their seat doesn’t need to be elderly or pregnant to respond considerately. They don’t need to share the same immediate experience to act because the other person’s physical difficulty is readily apparent and easy to imagine. That means that, even when the burdens experienced by two people are starkly different, mutual courtesy can bridge the gap, as long as both people understand the nature of that difference.

This premise works, mostly. However, an intractable problem arises when what appears to be a shared experience has unknowingly fractured beneath the surface. In such cases, conventional manners alone may be insufficient to resolve the conflict. That makes the pork bun problem an unusually demanding test of social etiquette.

So how, exactly, does this premise of shared experience break down inside a Shinkansen carriage?

Differences in our genes can shape how we perceive smells. The best-known example of this phenomenon involves the plant Coriandrum sativum (known as coriander or cilantro). While some individuals relish it as a refreshing herb, others find it deeply unpleasant, perceiving an odour reminiscent of soap or stink bugs. This divergence has been linked to a genetic variant near OR6A2, a gene involved in detecting smells. Such variation may affect the perceived intensity of the plant’s scent and also odour quality. The same leaves can seem fresh and herbaceous to one person but soapy or bug-like to another.

Courtesy Wikipedia

The pork bun controversy seems to reflect a similar form of olfactory diversity. It is known that pork fat can contain a volatile steroid compound called androstenone, which contributes to what is often characterised as an animal-like odour. Intriguingly, how people perceive this compound is strongly associated with variation in the olfactory receptor gene OR7D4. People inherit two copies of this particular smell-related gene, one from each parent. But those who happen to have inherited the more ‘sensitive’ versions of both copies tend to have a much lower threshold for detecting androstenone. For them, even a small trace of this compound is likely to be perceived as an unpleasant or animal-like odour.

Those who experience the smell of pork buns as something pleasant are not necessarily inconsiderate

Conversely, research from 2012 by Kathrine Lunde of the Norwegian Meat Research Centre and colleagues shows that people carrying different gene variants tend to perceive androstenone as less intense and less unpleasant. Over the course of primate evolution, vision became increasingly prominent, while smell mattered less. Eventually, many smell-related genes accumulated mutations and ‘switched off’. So how does this relate to ‘pork bun terrorism’?

Those who experience the smell of pork buns as something pleasant are not necessarily inconsiderate. Genetic studies suggest they may simply experience its aroma differently. To be clear, I am not claiming that androstenone alone accounts for the animal-like odour of pork buns. Nevertheless, the association between variation in the OR7D4 gene and androstenone perception offers a useful conceptual model for understanding the structure of the pork bun dispute on Japan’s Shinkansen.

If genetic variation in our smell receptors can alter how odours are perceived, then the foundational assumption that people in the same space experience the same odour in the same way is false. Furthermore, these genetic differences are mostly invisible and unknown to those who are (or are not) concerned about the smell of pork buns. Exposed to the same food odour, one passenger may experience an unpleasant, animal-like smell that triggers an involuntary emotional response, while another perceives an appetising aroma. Neither party is exaggerating. Instead, their brains are constructing qualitatively different perceptual experiences from the same source.

In everyday life, one person may bear a disproportionate sensory burden that others do not recognise. Noise disputes offer a clear parallel: for a person with misophonia, a disorder in which certain sounds may lead to a disproportionate emotional reaction, merely hearing someone chewing or breathing may provoke intense, involuntary distress. If we assume that everyone experiences the same sound in the same way, that distress – and any request for relief – may seem incomprehensible. Similarly, disputes over office thermostat settings often become intractable because we too readily assume that the same room temperature produces the same bodily experience in everyone.

Another problem is that, in each case, the less-affected person often decides what counts as a reasonable reaction. If the stimulus seems tolerable to them, they assume that it should be tolerable to everyone. The person who is more affected is then dismissed as overreacting, even though the burden is not equally shared.

Following this logic, it becomes clear that such conflicts cannot be adequately resolved by rigid rules, nor by demanding equal concessions from both sides.

Hidden biological differences may be undermining the assumptions that make social cooperation possible. Equipped with scientific knowledge, we have a responsibility to explain these hidden asymmetries so that we can comprehend each other’s lived realities. Scientific understanding does not automatically make people kinder, but it can show us where empathy is needed.

If people could genuinely imagine the profound distress their actions might cause, more passengers might voluntarily refrain from eating pork buns on the Shinkansen. At the same time, those who find the odour repulsive might come to see that their ability to travel in an environment free of strong food odours is partly sustained by other passengers’ restraint. That recognition might foster gratitude toward those who choose to forgo a small pleasure during the journey. Demands can then become requests, and restraint can be recognised as an act of consideration.

Some people will undoubtedly continue to prioritise their own pleasure even when they understand another person’s distress. However, once invisible burdens are made comprehensible, the conflict need no longer be reduced to a petty squabble between people labelled ‘hypersensitive’ and ‘insensitive’. Only then can the real work begin: making requests, showing consideration, expressing gratitude and, where necessary, designing appropriate rules and environments.

The next time ‘pork bun terrorism’ erupts on a Shinkansen, perhaps there will be a pause before the first bite or the first rebuke. Could etiquette begin in that pause, as two Shinkansen passengers realise that the same pork bun may be taking their noses on very different journeys?