We Humans Aren’t Rational

Why humans, despite what we think, may not actually make the most thoughtful decisions

Clap once. Clap three more times. Now, look in any direction and point there. Why did you pick that direction? Do you know?

If you do, wow, you’re impressive. If not, don’t worry, the majority of people who I asked about this (including me) don’t know either. But, why don’t we know? In a world where we are bombarded with decisions, why didn’t we think through this one any more? 

Well, I recently went on a deep dive into this, armed with my quintessential chips and iced tea. I voyaged across the whole web just for all of you – even reading some really complicated research papers.

Before I rattle off a bunch of papers and citations, we need to answer a fundamental question: How do we make decisions?

The prevailing theory in neuroscience is that there are two systems. The system I just tested, system one, is what I call your ‘monkey brain’. It makes snap judgments and forms instantaneous impressions (1). System two analyzes decisions and situations rationally and is much better suited towards complexity in decisions (1).

Here are some examples of each system’s thoughts:

1: Wow, a walk sounds so nice! I’ll walk to school today. Ooh, that was irrational of me. I might as well write an article about that.

2: Hmm, taking a walk right now seems pretty appealing, but I may tire myself out before my big math presentation and I didn’t pack a granola bar to refuel this morning. I may be better off taking the bus to save my energy. I’ll figure out my CHASM article topic after consulting a few lists I’ve made.

Guess which pattern of thought I followed when deciding what article to write: you guessed it, the monkey one.

System one engages your somatosensory cortex, the place in your brain that receives sense-based inputs, like tastes, smells, what you see, and decisions there are helmed by connections between groups of neurons (1). System two relies on the hippocampus and the prefrontal cortex, both places in your brain that head up working memory (1).

We don’t know much more about decisions; they are, like most of neuroscience, still being studied. However, this current theory illustrates how system one can grab hold of us and let us act before we think.

In a study published in 2018, researchers found that decisions can be made up to ten seconds before a person realizes they’re even making the decision (2). The researchers explored when decisions were made by measuring participants’ brain activity in the prefrontal and parietal cortices. In the study, participants were instructed to pick a button (either left or right), and people picked buttons before beginning to actually click them. 

Going a bit deeper into the science of decision-making itself and how the researchers measured that the decisions were being made, from the current research on the topic, the prefrontal cortex, an area of the brain controlling several executive functions, has a large impact on decision-making under free-will conditions [3].

Of all the parts of the prefrontal cortex, the ventrolateral prefrontal cortex, which controls goal-based response selection, and the orbitofrontal cortex, which is involved with sensory processing, emotion and memory, play the largest parts [4]. The parietal cortex, a part of the brain that controls body and spatial consciousness, helps make decisions about objects we can see [5].

This brings up all sorts of new questions, like whether we have free will over our decisions, or whether we’re biased when making big decisions. Also, how do we turn off system one and turn on system two? The answers to this have not yet been discovered, but these questions are being researched now. 


  1. Piore, A. (2019, November 19). This is how our brains make decisions. Discover Magazine. Retrieved September 26, 2021, from https://www.discovermagazine.com/mind/this-is-how-our-brains-make-decisions
  2. Soon, C. S., Brass, M., Heinze, H.-J., & Haynes, J.-D. (2008). Unconscious determinants of free decisions in the human brain. Nature Neuroscience, 11(5), 543–545. https://doi.org/10.1038/nn.2112
  3. Soon, C. S., Brass, M., Heinze, H.-J., & Haynes, J.-D. (2008). Unconscious determinants of free decisions in the human brain. In Nature Neuroscience (Vol. 11, Issue 5, pp. 543–545). Springer Science and Business Media LLC. https://doi.org/10.1038/nn.2112
  4. Rudebeck, P. H., Saunders, R. C., Lundgren, D. A., & Murray, E. A. (2017). Specialized Representations of Value in the Orbital and Ventrolateral Prefrontal Cortex: Desirability versus Availability of Outcomes. In Neuron (Vol. 95, Issue 5, pp. 1208-1220.e5). Elsevier BV. https://doi.org/10.1016/j.neuron.2017.07.042
  5. Zhong, L., Zhang, Y., Duan, C. A., Deng, J., Pan, J., & Xu, N. (2019). Causal contributions of parietal cortex to perceptual decision-making during stimulus categorization. In Nature Neuroscience (Vol. 22, Issue 6, pp. 963–973). Springer Science and Business Media LLC. https://doi.org/10.1038/s41593-019-0383-6