Picture this.

Friends have invited you around for coffee in the afternoon, and you know that this also usually involves delicious pastries, cookies or other energy dense foods that make the coffee experience even better, albeit calorie expensive.

Having decided to reduce the amount of energy-dense foods within your diet in order to eat within your energy budget, you plan to eat a healthy lunch that’s conducive to your goals prior to coffee.

“If I fill up on protein, vegetables and some healthy fats before afternoon tea I shouldn’t over consume the pastries/cookies/chocolates, in fact, I may not feel like any at all!”

Great idea! All of those options will help with filling you up well into your afternoons.

You arrive at your friend’s house, greeted by the comforting smell of baking cookies. As you sit down for tea, she brings in a well-arranged tray of your favourite cookies.

“Maybe just one, it would be rude not to,” you think to yourself after five excruciating minutes of eyeing them off. Next thing you know you’ve eaten five. Every time you grab one you have a voice telling you that you shouldn’t be eating anymore, but the lure of that warm cookie is too much.

Or perhaps you’re out for dinner. You finish your main course, full and satisfied. Then out comes the dessert menu and despite not being hungry you suddenly grow a second stomach and order one anyway.

Sound familiar?

Many other scenarios play out in a similar fashion. In fact, if it involves pizza then I’m yet to discover a conclusion that doesn’t end in me overeating.

But why does this happen? People don’t want to overeat. I’m yet to meet anyone who wants to frequently overeat, gain excess weight and develop ill health! And yet, they do.

Obesity rates clearly illustrate this. Despite our best interests being the opposite.

What this demonstrates is that there is a disconnection somewhere between what we want to do and what we ‘actually’ do.

What drives us to continue to eat when we’re not even hungry and when our goals are often in direct competition?

To answer this question, we need to go to the top, the big boss that’s making all the calls.

The brain.

According to Nobel prize-winning psychology researcher Daniel Kahneman, we can divide the brain’s thought processes into two distinct systems he call’s system 1 and system 2. (keeping it original, I like it).

System 1 processes are fast, effortless, intuitive, and non-conscious. They decide whether the cookies in the above example look and smell tempting.

System 2 processes are slower, effortful, rational and conscious. They’re the voice in our head that pipes up about the potential health and weight consequences of the cookies you’re eyeing off.

What this illustrates straight away is that the two brain systems can have competing goals. What’s important to understand, in the context of this article, is that system 1 (the non-conscious one) is often the more influential in our everyday lives. What this means is that much of our daily decision making is more impulsive and non-conscious then we’d like to believe.

This may help to explain why we may ‘want’ to make healthy choices and yet often don’t.

Why, when presented with broccoli or brownies, the latter tends to reign supreme in most scenarios. It’s this battle between system one and system two brain processes that partly explains why we overeat despite not wanting to.

But why would our brain have a function that causes us to overeat?

Scientists call it an evolutionary mismatch; a situation in which once-useful traits become harmful once they’re brought into a new, unfamiliar environment.

In this example, hardwired brain processes that helped our ancestors survive and reproduce are the very same processes that now drive us to overeat and gain body fat.

Consider the lifestyles of our hunter-gatherer ancestors from which our current brains have evolved. It was much more likely for our ancestors to suffer from being underweight than suffering from obesity. This was because food was less abundant and required reasonable amounts of energy expenditure to gather it, especially energy-dense options such as fatty animals or honey. It was this lifestyle that drove the hunter-gatherer’s brain to evolve with numerous systems to help ensure that they survived, thrived and reproduced.

What are some of the adaptions hardwired in our brains that cause us to overeat?

1) The reward system

Our brain instinctually values particular properties of food. Above all else, it values calories and the fat, sugar, salt and starch that typically identify as calorie dense foods. Consumption of foods rich in these properties results in a significant release of dopamine in the brain which acts as a reinforcing molecule. That is, it reinforces the behaviour that leads to dopamine release, in this case consuming an energy-dense food. This motivates us to seek these foods in the future and increases our recognition of cues that will likely lead us to achieve this. Over time certain foods become especially rewarding to us, increasing cravings as the brain strengthens our motivation for these foods.

In short, based upon thousands of years of evolution we’re deeply motivated to seek out energy-dense foods as our brain recognises these as highly valuable and rewarding because they signify survival and reproduction. Fast forward to today and we’re constantly surrounded by foods that fit these criteria, making it increasingly hard to regulate our intake.

2) Economic choice system

Our brains are great at finding the best ‘deals’.

Our ancestors had to weight up the cost of certain foods based on their energy return, the energy expended to acquire it and the time it took. Scientists have termed this the optimal foraging theory (OFT), which can fairly accurately predict what energy sources animals and hunter-gatherer communities would likely choose. What they have found is that a food’s value is roughly determined by its calorie return rate. Our hunter-gatherer ancestors rarely bothered collecting low calories foods as it doesn’t make sense to expend 200 calories to obtain 50 calories worth of food. (Why we’re not driven to jump in the car at midnight and drive to a service station for baby spinach).

Do you think that our ancestors were preaching moderation and portion control? Absolutely not. In fact, it was likely the opposite. They were at times gluttonous, especially when energy dense food options presented themselves. There are accounts of the Hadza tribe of Tanzania drinking honey like it was water or eating huge quantities of fatty meat.


It meant increased chances of reproduction and survival.

For most of human history our instinctive drive to seek large amounts of energy-dense foods rich in sugar, fat or protein were well aligned with their best interests.

Unfortunately, despite a radically different environment, our brains are still attuned to excellent “deals” (have you ever seen how quickly people flock to free food?). While our ancestors may have encountered these deals rarely, we are bombarded with them multiple times a day.

3) Immediate gratification vs. future self

There’s yet another reason why we often choose to eat despite our best interests. When it comes to a decision that pits our current self against our future self, more often than not, we make self-destructive choices.

Going back to our scenario at the start, most people will agree that a warm, slightly soft chocolate chip cookie is delicious. Most of us will also recognise that they’re not a typically “healthy” option. The benefit of the cookie is its reward value. It just tastes sooooo good. You experience this warm cookie goodness right then and there.

The costs of those cookies though, they’re all incurred by your future self. Potentially one minor step closer to obesity and ill health. Once again this illustrates our competing brain systems. Our non-conscious brain doesn’t care about the future or our longevity. It just wants the cookie right now. Our conscious, rational brain understands the value of the future, of health and longevity. Which system comes out on top can depend on a psychological trait known as delay discounting, or how much we undervalue a future reward versus an immediate one. In other words, how much an individual values their future self versus their present self. People who value their future selves highly also tend to value long-term goals like leanness and health. For someone like this, a slimmer body next summer is worth more than the chocolate chip cookies. Numerous studies have shown that individuals who steeply discount the value of future rewards are more likely to have obesity, gambling problems and smoke.

Why do we make these self-destructive decisions?

Life was unpredictable for our ancestors. Rarely was their future guaranteed. They lived in a dangerous environment, where life expectancies were incredibly low by today’s standards. If you weren’t certain you’d be alive next year, then it makes sense to place more value on the now.

Compare that to our lives today, and the future is much more certain. It makes a lot more sense to value our future selves just as much as our present selves, unfortunately, our brains still playing catch up.

We’ve covered three ways our brains have evolved that can contribute to overeating. Reading this you may be feeling a little deflated. If our brain has evolved to work against our best interests, what can we do?

Rest assured there are measures and tips we can apply to counter our brains evolutionary traits. I’ll cover some of these in part 2 of the series which I promise will be short and sharp.

Stay tuned.

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Guyenet, Stephan J. The Hungry Brain: Outsmarting the Instincts That Make Us Overeat. New York: Flatiron, 2017. Print.

Kahneman, Daniel. Thinking, Fast and Slow. New York: Farrar, Straus and Giroux, 2015. Print.

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Hawkes K, O’connell JF, Jones NB, Oftedal OT, Blumenschine RJ. Hunting income patterns among the Hadza: big game, common goods, foraging goals and the evolution of the human diet [and discussion]. Philosophical Transactions of the Royal Society of London B: Biological Sciences. 1991 Nov 29;334(1270):243-51.

Pyke GH. Optimal foraging theory: a critical review. Annual review of ecology and systematics. 1984 Nov;15(1):523-75.

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MacKillop J, Amlung MT, Few LR, Ray LA, Sweet LH, Munafò MR. Delayed reward discounting and addictive behavior: a meta-analysis. Psychopharmacology. 2011 Aug 1;216(3):305-21.