Why you eat more than you think: Delboeuf's Illusion

Optical illusion that manipulates your rations

🏷️ Categories: Decision making and biases.

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In 1865, psychologist Franz Joseph Delboeuf discovered a curious optical illusion: when you see a circle surrounded by another, your brain perceives its size differently depending on the size of the outer circle.

• If the outer edge is close, the inner circle appears larger.

• If it's farther away, it seems smaller.

But the surprising part isn't the illusion itself.

The surprising part is how it affects your eating habits—and how it pushes you (without you realizing it) to eat and buy more food. If you want to understand how an optical illusion can influence your diet, by the time you finish reading, you’ll understand why your brain gets it wrong every time you fill your plate. And most importantly...

What you can do to avoid it.

1. Your brain doesn’t measure portions—it compares space.

When you serve food, your brain doesn’t evaluate “how much there is.”

It evaluates “how much there is relative to the space occupied.”

A large plate creates visual emptiness between the food and the rim. Your brain interprets the portion as small… and compensates by serving more. On a small plate, the rim is close to the food. It looks like more. So you serve less.

This isn’t just theory—it’s been measured.

A study by Van Ittersum and Wansink found that plate size has increased by 23% since 1900. The result? An average of 50 extra calories per meal. That’s the equivalent of two extra tablespoons of cooked rice or pasta per plate. It may seem small, but over a year, it adds up to more than 2 kilos of weight gain.

And all that… from a visual illusion.

2. The silent influence of design

Plate size isn’t the only thing that matters. The rim plays a role too.

• A wide plate rim makes the food look 10% larger.

• A colored rim enhances perceived portion size by 3%.

And if the color of the food matches the plate (white pasta on a white plate), we tend to serve more. But if there’s strong contrast (white pasta on a dark plate), we serve less (McClain et al., 2014).

Why?

Because contrast grabs attention. It makes your visual system scream: “Hey, that’s a lot!” And you serve less food because the amount is more noticeable.

And all of this happens without your awareness.

3. Images lie (and we buy more because of it)

Here’s the most curious part of the effect…

The same optical illusion is used by food brands to make you buy more. The trick is in the packaging. A group of scientists ran an experiment: they showed food packaging with images of food on small plates (to make it look bigger), such as cereal boxes and frozen pizzas (Petit et al., 2018).

The result?

Food looks more appealing and plentiful when shown on smaller plates.

Images of food (especially calorie-dense ones) activate brain areas linked to pleasure and reward. Your brain anticipates how satisfying the experience will be. And it wants more.

4. Redesign your environment

The obvious question: Can we use this illusion to our advantage?

The answer is yes. Here are 2 science-backed tips:

• Use smaller plates: Reduce your portion size without even noticing. You’ll feel like you ate the same (or even more) just thanks to visual perception.

• Create high contrast: Serve light-colored foods on dark plates (and vice versa). Your portion will look bigger, and you’ll feel fuller.

These small changes are nudges—they don’t forbid anything, but gently guide your behavior toward better choices. Toward better habits.

And they work.

5. The powerful and silent influence of your environment

The Delboeuf illusion is a reminder of an uncomfortable truth…

Our environment shapes our behavior without us realizing it.

Every time you choose a portion, you’re not guided purely by hunger. You’re being influenced by plate size, rim color, visual contrast, and a long list of subtle factors. But knowing about this illusion gives you a huge advantage.

You can redesign your environment to work for you, not against you.

Interested in exploring more about how your environment shapes your habits and decisions?

I’ve written more articles that go deeper into just that: how to live with intention, design environments that support you, and change your habits without relying on willpower. Here’s a curated list with the best ideas:

1. How to change habits (without needing willpower or struggle)

2. If you want to change, change your environment: The Vietnam’s soldiers case

3. How to create an environment that drives you.

✍️ Your turn: How many decisions do you make each day that are actually already made… by design?

💭 Quote of the day: “By presenting food in a smaller container, thus creating the illusion of a relatively larger portion, people show a greater purchase intention.”
— Petit, Velasco & Spence

See you in the next one! 👋

Referencias 📚

1. McClain, A. D., Van Den Bos, W., Matheson, D., Desai, M., McClure, S. M., & Robinson, T. N. (2013). Visual illusions and plate design: the effects of plate rim widths and rim coloring on perceived food portion size. International Journal Of Obesity, 38(5), 657-662. URL

2. Petit, O., Velasco, C., & Spence, C. (2018). Are large portions always bad? Using the Delboeuf illusion on food packaging to nudge consumer behavior. Marketing Letters, 29(4), 435-449. URL