Home HealthPsychology How Cognitive Biases Make Us Eat Bad Diets

How Cognitive Biases Make Us Eat Bad Diets

by Joseph Whittaker

Cognitive biases are the inbuilt flaws in our rational thinking process. They mislead our brains and cause us to draw faulty conclusions. No other area of human knowledge is there more disagreement than nutrition. Anyone who reads about it will quickly find themselves in a mass of contradicting viewpoints, dogmatic cults and misinformation. The cognitive biases that shape our thinking can help explain this informational mess, called nutrition.


The Nutritional Information Mess

Right now the number of different diets is staggering:

paleo, primal, raw food, vegan, vegetarian, low fat, metabolic typing, Weston A Price, Mediterranean, South Beach, Atkins, low carb, ketogenic, carb cycling, zone, weight watchers, macrobiotic, GAPs, FODMAPs, etc.

Too Much Information!

Too Much Information!

All these diets claim to be based on science and evidence. Furthermore, the scientific method aims, in so far as possible, to be objective and remove bias. So why do the proponents of these diets come to different conclusions? Especially if they are (supposedly) all using the same evidence.

I will look at 3 issues in the health world: the lipid hypothesis, veganism, and the misinterpretation of evidence by diet books; and explain how our faulty brains have contributed to their success.


But, why are our brains faulty? ‘I thought we were pretty rational‘ I hear you say. Let me explain…


Why Do We Have Cognitive Biases?

Contrary to popular belief, we are not completely rational beings and we do not act like organic computers. In fact, when we think rationally we are subject to a number of inbuilt biases. These innate cognitive biases were shaped by evolution to provide shortcuts for our thoughts, so we could quickly draw conclusions about how things worked.

Our brain evolved most during our time as hunter-gatherers and is tailored to that environment. We needed to make quick decisions to avoid predators and be effective hunters. In addition, our environment was less complex and more stable, than the modern world. Therefore, shortcuts in thinking were evolutionarily effective. However, we are now faced with more complex and different problems, and these cognitive biases no longer serve their intended purpose. Instead, they misguide our thinking and lead us to false conclusions.

However, the good news is that many of these are known and they are uniform across people. In the words of the behavioural economist Dan Ariely, we are ‘Predictably Irrational’.


1. Why People Still Can’t Eat Butter?

In the 1960s the consensus was that saturated fat was bad and this idea peaked in the 1980s. The current evidence clearly shows saturated fat is not bad for us, but this idea still persists as saturated fat and foods that contain it (animal foods) are generally viewed as unhealthy. Moreover, there are still nutritionists and doctors peddling this false idea.

Thomas Kuhn in his famous book The Structure of Scientific Revolutions explained that science is not the linear accumulation of evidence, but instead goes through cycles of paradigms. One paradigm is upheld by scientists until a new superior paradigm comes along and displaces it. In his book, he quotes Max Planck, the originator of quantum theory:

A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grow up that is familiar with it.


The Lipid Hypothesis Just Won’t Die!

Old school proponents of low-fat diets cannot bring themselves to change their minds and admit that they were wrong. This is partly due to the status quo bias, which states people want to uphold the current state of affairs. In a famous experiment, researchers asked consumers about their preferences on 6 electrical plans, with varying rates of reliability and price. The consumers overwhelmingly chose to stay with their existing plan, even though the other options may have been a better choice logically (1).

The more we invest in an idea, the more we seek to uphold it. Human beings are naturally loss averse. Research consistently shows we value avoiding losses, more than acquiring gains (2). In other words, we don’t want to lose what we already have. In terms of belief, we avoid change and stick to our old views. There is an addition bias called the existence bias, which states that people use the fact that something exists, as proof that it must be good, longevity increases this effect (3). Therefore, we think beliefs that have been around for a long time, must be correct since they have such staying power. In our example, the lipid hypothesis (saturated fats cause heart disease) has been around so long, it just won’t go away!


2. Why Do People Think Vegan Diets Are Healthy, When Science Says The Opposite?

Veganism, raw food, and plant-based diets are everywhere. Veganism has increased by 350% compared to a decade ago, with 1% of the UK population identifying as vegans (4). There are similar increases worldwide. However, the scientific evidence does not support a vegan diet, so why are so many people turning to it?


Vegan History

Veganism has a long history of being associated with spiritualism. Many early Indian philosophers and Buddhists took up the practice, citing animal welfare reasons. Throughout the 20th century, it has gradually gained steam, due to the idea that saturated fat and animal foods containing it were bad (the lipid hypothesis). However, there has been an explosion of interest in veganism, in the 2010s.


The Bubble of Veganism

The recent mass interest in veganism is not based fact. There has not been some new groundbreaking research published that supports veganism. Moreover, popularity does not equate to goodness. For instance, Justin Bieber is one of the most popular artists today, but few would argue he writes the greatest music.

The rise of veganism is due to the bandwagon effect. As more people turn vegan, others see this and also turn vegan. Then other people see that and turn vegan, which results in an exponential rise in veganism. People are jumping on the bandwagon, due to our tendency to follow the behaviour of others. They are not sifting through the literature and rationally deciding this is the best course of action.

Vegan Popularity Is Akin To A Stock Market Bubble, bandwagon effect, one of the cognitive biases.

Vegan Popularity Is Akin To A Stock Market Bubble

It is like a stock market bubble where the price of a certain commodity gets overvalued and causes more people to buy it, which in turn causes even more to buy it. Human beings can be subject to a herd mentality, in which one follows the group and those who lead it. In a novel experiment conducted at the University of Leeds, researchers asked people to walk around a hall undirected (5). However, a few participants had been instructed to walk a specific route, and soon all the others followed them. The lead researcher Professor Kruse noted:

In most cases the participants didn’t realise they were being led by others. (6)


Conformity, Mere Exposure and The Rise of Veganism

One reason why we follow the behaviour of others is due to our strong need to conform. In 1951, psychologist Solomon Asch conducted a pioneering experiment on conformity (7). He gathered 8 subjects in a room and gave them 1 card with 1 line drawn on it, and another card with 3 lines drawn on it (see picture below). The subjects had to match the 1 line card, to the line of the same length on the other card. The answer was obvious, but 7 of the subjects were actually actors and deliberately gave the wrong answer. The one true participant gave the incorrect answer 37% of the time, despite it being clear it was the wrong answer. They denied their rationality and ‘went along’ with the others, due to the social pressure to conform.

The 2 Cards from Solomon's Experiments, Conformity, one of the cognitive biases

The 2 Cards from Solomon’s Experiments

In addition to conformity, the mere exposure effect can also help explain the rise in veganism. This effect states that the more someone is exposed to something, the more they like it. In other words, we like things that we are familiar with. In the 1960s, the pioneering psychologist Robert Zajonc showed that people rated stimuli more highly when they had been exposed to it before, than when they had not (8). He used stimuli such as words and sounds, and found the more people were exposed to it, the better they liked it.

The idea of veganism is everywhere: from juice bars to celebrity vegans. We have become familiar with the idea and therefore like it.


3. The Misinterpretation of Evidence by Diet Books

The scientific literature should be able to objectively show the answer to: what is the best diet? (Even if that answer is highly nuanced and variable due to individual differences). However, we see that all these different diets use science to justify their approaches. Therefore, diet proponents must be misinterpreting or cherry-picking the evidence, since not all diets can be correct. So, the question is why are they doing this?


The Reason Why Diet Books Are Biased

A typical diet book starts with the author recanting their personal health or weight struggles. Then, after some searching, they fall upon the magical diet that solves all their problems. The fact is, different diets work for different people and this is due to a myriad of reasons (genetics, carb tolerance, gut biome, current health conditions, digestive health, etc). So that specific diet probably did work for the author, due to various individual reasons. However, the author then seeks to generalise their individual results to the population at large.

As they search the literature they end up mostly looking for and reading information that confirms their point of view. This is known as confirmation bias, which is well studied in psychology (9). Furthermore, personal experience can often be difficult to accurately interpret. We tend to patterns where none exist, this is known as illusionary correlation (9).

In addition to this, even their interpretation of the evidence can be biased. An experiment conducted at Standford University, had students read studies on capital punishment (10). Initially, the students changed their minds in favour of whatever the study first study concluded, but as they read more detailed versions of other studies almost all reverted back to their original opinion, regardless of the evidence. People also place a higher standard on evidence that goes against their chosen position, this is known as disconfirmation bias (11).


The Reason Why Readers Are Biased

After the book is published, we read it. However, the evidence is framed in a highly subjective and often emotive manner. How the information is framed (i.e. how it is presented), is well known to influence the reader’s perception of the evidence, this is known as the framing effect. An example of the framing effect is presenting treatments in terms of relative risk instead of absolute risk (explained here – under ‘Statistical Terms Explained’) (12). Overall, this gives the reader a skewed version of the evidence.


Key Points

  • Our brains have not evolved to be completely rational and objective.
  • Instead, we use many shortcuts in our thinking, to make quick decisions and conclusions. These shortcuts are known as cognitive biases.
  • The Lipid Hypothesis has been disproven, but still persists due to the status quo bias, which states people prefer not to change their beliefs.
  • Moreover, the longer an idea has been around, the greater it is seen to be, due to the existence bias.
  • The recent explosion of veganism can be seen as a stock market bubble.
  • The reason for this rise is due to the bandwagon effect: popular beliefs become exponentially more popular.
  • The bandwagon effect is due to our need to conform and follow the herd.
  • Diet books often start with personal anecdotes of success, which leads the author to selectively interpret the evidence in favour of their viewpoint (confirmation bias).
  • Diets books are written in a subjective and emotive manner, which frames the evidence in a certain way.
  • This framing influences the reader’s perception of the evidence.


Photo Credit: Asch experiment by Nyenyec, CC BY-SA 3.0;

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1 year ago

Not all vegan diets are the same, but appropriately planned vegan diets are healthy, there’s plenty of evidence : https://youaretheirvoice.com/resources/the-clear-consensus-print-friendly-en.pdf
As for people turning vegan as a way to conform, how can it be? There are less than 2% vegetarians in the world and maybe 1/4 of them are vegan. People are using conformity as a way to justify not to go vegan…

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