Most people have heard of the Prisoner’s Dilemma; fewer have heard of the Unscrupulous Diner’s Dilemma, although you most certainly have lived it. 

In game theory, the Unscrupulous Diner’s Dilemma is a variation of the Prisoner’s Dilemma. Imagine that a group of people go out to eat. Before ordering, they agree to split the bill equally. Each diner now has a choice: order something modest, or something pricier? If you order something expensive, you bear only a fraction of the extra cost while everyone else shares the rest. But if everyone thinks the same way and orders what is most expensive, everyone ends up worse off than if they’d each just ordered what they felt like having.

In a 2004 study, behavioural scientist Uri Gneezy and his collaborators sought to test the Unscrupulous Diner’s Dilemma in an experiment. They recruited groups of six strangers to dine at a restaurant, telling each group a different thing: one group was told to pay for their own meal, one split the bill equally, and the last group was told that their meal was free. The question was simple–would people order differently depending on how the bill was paid?

Before the experiment began, diners were asked which payment mode they’d prefer. 80 percent said they’d rather pay individually. They probably felt it was the fairer arrangement.

Comparing eventual ordering behaviours across the groups, those placed in the split-evenly group ordered significantly more than those paying individually. Unsurprisingly, those who had their meals paid for ordered the most of the three groups.

Most of us, if asked, might say the same thing as that 80 percent—we want to pay individually, because that feels fairest. And yet in real life, we often default to splitting equally because it’s simpler and less fraught. Or we pay individually when the dishes are clearly separable. Either way, we reach for whichever method feels most fair at that moment. 

But what we say we want and what we actually do when the bill arrives is often not the same thing.

I once lived a version of the dilemma at an office dinner. A few of us decided to have dinner together after work—no occasion, just dinner. We ordered the usual—har jeong kai (prawn paste chicken), sambal kang kong, yam basket, hotplate tofu, some vegetables. And chrysanthemum tea. It felt like enough.

Then someone suggested chilli crab. Only a few people wanted to eat it (we were all in officewear and the whole shell situation felt like too much hassle). But some of us didn’t mind the sauce if we also got some fried mantous. The same person went on to also order a jug of beer (“Must have beer with crab mah…”). A few people drink, but not all. By then, the food had started arriving, people were busy talking, so no one made a fuss.

And when the bill came, we split it equally. It was just easier. But I realised that two colleagues had touched neither the crab nor the beer. They paid as much as the rest did. I thought about saying something. But then I didn’t. It wasn’t a lot of money and these were people I’d see the next morning. 

What happened that evening was the Unscrupulous Diner’s Dilemma in its most recognisable form. I don’t think anyone planned on taking advantage of the rest of us. Splitting the bill equally was just assumed. And so why wouldn’t you order the crab and beer? You could call this an example of moral hazard—the tendency to make different choices when someone else bears part of the cost, whether that’s an insured rental car or a shared restaurant bill. You pay only a fraction of the true cost of your own consumption since everyone shares the whole bill. The two colleagues who had neither crab nor beer ended up subsidising both. Gneezy’s strangers in a restaurant, us at our zi char stall in Singapore–the same logic, the same outcome.

Economics explains why I didn’t say anything. The cost of raising it—the awkwardness, the implicit accusation, the risk of being that person who made an issue of a work dinner—outweighed what any of us was actually owed. Ronald Coase termed this transaction costs in the 1930s, work that later won him the Nobel Prize in Economics in 1991. It’s the friction involved in any exchange—not just money, but time, effort, and goodwill. When the transaction cost of fairness exceeds the unfairness itself, people absorb the cost and let it slide. 

We found a workaround, of sorts. In the weeks after, some of us started gravitating toward places where everyone could order their own thing—a plate of pasta, a rice bowl, economy rice. Things with a clear price and a clear “owner”. When everyone orders individually, the moral hazard disappears. Problem solved.

Or so it seems.

Now picture a late-night prata shop somewhere in Singapore. A group of friends are having supper. Everyone has ordered their own thing—prata, mee goreng, roti john, tehs all round. Midway through, another friend turns up. No one seems surprised that he’s late. He pulls up a chair and orders a teh halia. And then starts to help himself to a bit of everyone’s food—half a prata here, some roti john there. No one says anything; this clearly happens all the time. 

When the bill comes, each person pays for what they ordered. The latecomer ordered a teh halia. So he pays just for a teh halia.

Here, the table tried to do it “properly”. Everyone paid for what they ordered. This is the division that’s supposed to feel most precise, most fair. No equal split, no assumed subsidy. And yet it still produced what might feel like an unfair outcome. 

Part of this is due to an informational gap. The food was shared and passed around. Nobody kept track of who ate what and by the time the bill arrived, that information was simply gone—the mee goreng eaten and only prata crumbs left. Economists might liken this to the monitoring problem: when consumption is shared and informal, individual contributions simply can’t be observed or verified. 

But part of it is something else entirely. The table has probably accepted who this person is to them. He’s always like this. He’s been like this for years. And he’s still welcomed, still in the group chat. Sure, he only pays for the teh halia, but the table has collectively decided that he’s more important than the cost of the occasional extra bite of roti john. 

So at this prata shop, the table absorbs this cost. Someone mutters, “Aiyah, he’s always like that one lah.” And people move on. What the table chose, in the end, wasn’t fairness; it was him. Wanting fairness and choosing it turns out to be two different things.

There are apps for this now, Splitwise being the most popular. You can log every chilli crab, every teh, every half-eaten prata, and it will calculate, to the cent, who owes what to whom and when. The thing is, the problem Splitwise solves is the maths, but that was never the hard part. What it can’t decide for you is whether the teh halia guy is worth the roti john, or if some costs are absorbed not because they’re small but because that one person looms large in your life. 

Everyone has their own version of these tables, your own moment of noticing something “unfair” and deciding whether or not to say something. For me, it was supper and crustaceans; for you, it might be a round of drinks with satay and BBQ sting-ray.

This is what makes bill-splitting genuinely hard—not just because of the economics of it, but because each table has its own informal rules, its own history, its own version of what counts as fair. 

Months after the office dinner, we drifted back to the same coffeeshop—same colleagues, same table. Someone ordered fish head curry to share. We split it equally.

Serene Koh is a behavioural scientist and director of the Behavioural Insights Team in Singapore. She also teaches behavioural science at the National University of Singapore. 

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