Why Do You Finish Your Plate: Waste Aversion and Sunk Cost Fallacy

Imagine the following scenario:

You are having lunch at a restaurant. After eating 2/3 of your plate, you feel quite full already. If you keep on eating, you will very possibly feel too full. No one is willing to eat the leftover and you cannot take it home. Will you finish the 1/3 that is left? 

For most economists, they would say no and suggest you not doing so too. The price that you have to pay or have paid is already a sunk cost, which means that you cannot recover the cost. Eating more or less would not make you pay more or less. After you order that dish, all you should do is to maximize your net utility (the happiness/satisfaction from the consumption of the plate). You should look at the future, enjoy the dish as much as possible and avoid the other way. Therefore, in this scenario, you should stop eating because you would feel too full and your utility would decrease otherwise. This concept can be illustrated using hypothetical and conceptual units, utils, as follows:

The utility of having the first 1/3 is higher than the utility of having the second 1/3 because of diminishing marginal utility. If the dish is tasty, then you should enjoy the first few bites a lot but the extra satisfaction from every bite should decrease over time.

But quite many people would keep on eating because:

1) they have already paid for the meal and they think they can cover the cost (the price) by eating the whole thing, and/or 

2) they would hate to see the food on their plate ending up in the dumpster. It would make them feel guilty. This is more common in some cultures. The Japanese culture would be one of them. 

The first one is called "sunk cost fallacy", and the second one is what I call "waste aversion" (very straight-forward). The truth is that resources for that dish has already been used. Finishing the steak would not reincarnate the cow; nor would finishing the broccoli bring it back to the farm. Whether it ends up in your stomach or dumpster, the resources have already been used. The food ending up in the dumpster does not incur a new cost to the world but if it ends up in your stomach, it would cost you utility (lower satisfaction). If you want to avoid waste, just be careful of what you order next time. If you feel sorry for wasting food while people in poor countries are in famine, donate money to a charity that uses your donation well.

Whether it is rational or not, it is good for us to be able to describe the psychology of waste aversion. This table illustrates the concept. Blue is where economist think we can maximize our utility and yellow is where someone with moderate or strong waste aversion think they can maximize their utility.



For an economist, they only look at the consumption utility and say that you should stop when it is maximized. But in the perspective of many homo sapiens, waste utility is also taken into the account. If they order a dish and not eat it at all, then they will leave with negative utility because of the sunk cost fallacy and waste aversion. If they eat only 2/3 of the plate, they will get 10 utils from the consumption but the waste aversion would cost them -5 utils so in total, they will only have earn 5 utils from the whole experience.

As the person eats more and more, the cost of utility of waste aversion decreases by a bigger margin. This is clearer in the graph. 

Maybe it is our nature to tend to hate wasting food. It was part of our ancestors' survival to avoid any waste and eat every bit of the food they gathered or hunted. But waste aversion can still be reduced by altering your perspective. Educate yourself and tell yourself to not feel guilty about it.

Note:

- Leaving food on your plate probably involves loss aversion too. It may make you feel like you lose you money to the restaurant because you eat less than what you pay for. 

- I admit that even when I fully understand what sunk cost fallacy and waste aversion are, I still can't help but naturally feel a little guilty for not finishing a plate or a drink.

- The concept of waste aversion is my original hypothesis based on my observation. I would love to do research on this topic in the future. If you know there are already scholars who have studied this, please let me know.