System 2 is expensive to run, so your brain avoids using it
In the last essay, I introduced you to System 1 and System 2 – your brain’s fast, automatic thinking versus its slow, deliberate reasoning. System 2 is the one that can actually catch errors and make better decisions. So here’s the obvious question: if System 2 is so much better at thinking, why don’t we use it more often?
The answer is System 2 is expensive to run. Daniel Kahneman – Nobel Prize winner, psychologist, the guy who mapped all this out – calls this the Law of Least Effort.
Your brain will always choose the path that requires the least mental energy. Not sometimes. Always. This isn’t a bug in your cognitive system. It’s a feature.
Here’s why.
Your brain represents about 2% of your body weight but consumes roughly 20% of your total energy. That’s already a massive metabolic cost just for baseline operations. When you activate System 2 for effortful thinking, that cost spikes even higher. We’re talking measurably increased glucose and oxygen consumption. Your pupils literally dilate. Your heart rate increases. In some cases, people report feeling mild physical discomfort.
Try this right now: calculate 17 times 24 in your head without writing anything down.
Feel that strain?
That’s what cognitive effort actually feels like. Your brain is treating this task like an expensive operation that needs to be justified. Now compare that sensation to recognizing your dog’s face or understanding these words as you read them.
System 1 handles those tasks with zero noticeable effort.
Your brain notices this difference. Over thousands of small decisions about where to allocate mental resources, it develops a strong bias: avoid the expensive operation whenever possible.
The Law of Least Effort shows up everywhere once you start looking for it. You’re scrolling through a news article and hit a dense paragraph full of statistics and technical terms. What do you do? If you’re honest, you probably skim it, maybe pick out a few key words, and move on. You tell yourself you got the gist. System 2 could force you to slow down, parse each sentence, check if the statistics actually support the conclusion being made. But that would require serious effort, so it doesn’t activate.
Or you’re in a meeting and someone proposes a solution to a problem. The first option sounds reasonable enough. You could spend mental energy analyzing whether it’s actually the best option, considering alternatives, thinking through second-order consequences. But the first option is already on the table and nobody else is objecting. System 2 could engage, but why bother? The path of least resistance is to just go along.
Here’s where it gets really problematic.
The situations where you most need System 2 are often the situations where it’s least likely to show up. High-stakes decisions. Complex problems. Emotionally charged topics. These are cognitively demanding scenarios where your mental resources are already strained. The Law of Least Effort pushes you toward simpler mental processes – gut feelings, familiar patterns, whatever System 1 serves up – precisely when careful analysis matters most.
Someone challenges one of your deeply held beliefs. Your immediate response isn’t “Let me carefully examine whether I might be wrong.” It’s defensive. System 2 could force you to genuinely consider the challenge, but that would mean admitting you might need to change your position. That’s cognitively expensive and emotionally uncomfortable. Much easier to let System 1 generate a quick dismissal and move on.
Rolf Dobelli documents this pattern across dozens of thinking errors in “The Art of Thinking Clearly.” We accept weak arguments because scrutinizing them feels laborious. We maintain contradictory beliefs because reconciling them would require effort. We follow the crowd because independent thinking is cognitively demanding. The laziness isn’t random. It’s systematic.
What makes this particularly dangerous is that you don’t experience it as laziness. You don’t feel yourself making a choice between “thinking carefully” and “taking the easy route.” The easy route just feels right. System 1 generates an answer that seems perfectly adequate, and unless something specifically triggers System 2 to engage, that’s the answer you go with. You remain completely unaware that you just chose efficiency over accuracy.
Your brain isn’t malfunctioning when it does this. It’s doing exactly what it was optimized to do. For most of human evolutionary history, quick decisions about immediate threats were far more valuable than careful analysis of abstract problems. The ancestor who could instantly recognize a predator and run survived. The ancestor who stopped to carefully analyze whether that rustling in the bushes was really a threat often didn’t. We inherited the brains of the ancestors who survived.
But we’re not on the savanna anymore. The threats we face aren’t charging at us from the bushes. They’re embedded in complex policy decisions, financial instruments we don’t fully understand, medical information we can’t properly evaluate, and political claims we accept without scrutiny. The same mental efficiency that kept us alive in one environment now regularly leads us astray in another.
This is why critical thinking has to be taught and practiced deliberately. The infrastructure for clear thinking exists in your brain, but it won’t activate on its own. System 2 has to be triggered, and it has to be worth the metabolic cost. You have to build habits that make effortful thinking more automatic.
You have to create environments that make cognitive shortcuts less tempting.
The first step is just acknowledging the reality. Your brain will always prefer the easy answer. System 2 will always be reluctant to do its job. You can’t eliminate this tendency, but you can learn to recognize when it’s happening and compensate for it. You can get better at noticing when you’re coasting and forcing yourself to engage more deeply.
Good thinking is hard work. Your brain knows this and will resist that work at every opportunity. If you want to think better, you first have to accept that your brain doesn’t want to think harder. That’s not a moral failing. It’s just biology operating exactly as designed.
In the next essay, I’ll talk about the specific mechanisms System 1 uses to satisfy this craving for cognitive efficiency. The mental shortcuts – heuristics – that allow your brain to make fast judgments without burning all that expensive glucose. These shortcuts are where the Law of Least Effort translates into concrete, predictable errors.